This helps in logging up the requests made by ipython_notebook service

Following changes :-
1. More comments and description.
2. Useless empty display_response removed.
3. Use of MagicInfo class and then MAGICS dictionary to ease up with lookup.
4. Error handling more robust.
5. Add restriction message on Jupyter frontend for user to not change module objects explicitly
6. For successfull request to server, use mime_type returned from result only
7. Add message in custom.js to not dynamically alter imported module for users
8. Ease in adding new magics
9. Refactored erp5 kernel to enter required magics ['user', 'password', 'url', 'reference'] in any possible combinations
10. Exception handling for content which aren't json
11. Docstring for ERP5Kernel class
12. Handle cases for server side error in ERP5Kernel
13. Add display_response parameter
    'display_message' parameter would be used to check if there is a need to display 'message'
     after making a successful request to erp5 or not.
     Useful for magics such as '%my_notebooks' for which we don't need to show message cause we are
     already getting some content from erp5 for this magics.
14. rename response parameter to message.

IPython Notebook: Explicitly add environment variable around wrapper and use ipython directory inside instance in env

IPython Notebook: Add dynamic-template-base section for common jinja related file section and extend them with this section

IPython Notebook: Add download-file-base section which would be extended by sections requiring common usage

Use spinal-case('-' separated) instead of snake_case('_' separated) in section names.

If one wants to check URLs on UNIX-sockets, there is no full URL schema
in curl for this, but the following has to be used instead:

    curl --unix-socket /path/to/socket http:/<url-path>

For this to work, one can do e.g. the following trick:

    [promise-unicorn]
    recipe  = slapos.cookbook:check_url_available
    url     = --unix-socket ${unicorn:socket}  http:/

but then generated promise scripts fails this way:

    ./etc/promise/unicorn: line 7: [: too many arguments

via quoting $URL in emptiness check we can support both usual URLs and
urls with --unix-socket prepended trick.

/reviewed-by @cedric.leninivin  (on nexedi/slapos!31)

In gitlab SR a service I need to check - gitlab-workhorse, returns 200
only when request comes to some repository and authentication backend
allows it.

Requiring access to repositories is not very good just to check if the
service is alive, and also auth backend can be not alive, and initially
there are no repositories at all. So gitlab-workhorse is checked to be
alive by pinging it with non-existing URL and expecting 403.

For this to work we need to allow clients to specify expected HTTP code
instead of previously hardcoded 200 (which still remains the default).

/reviewed-by @cedric.leninivin  (on nexedi/slapos!31)

  The final file were not a valid json sometimes as some entries '0.0.0.0' or '::' were been ignored
  and an additionalcomman was added on the wrong place.

Both SPDY an gzip_static are needed for upcoming GitLab SR:

  - GitLab uses SPDY in its https nginx configuration, and
  - prepares compiled assets in pre-gzipped form on filesystem

both modules are off by default and need to be explicitly enabled with
corresponding directives, so this should not affect already used nginx
configurations.

/cc @kazuhiko, @jerome, @gabriel
/reviewed-by @rafael  (on nexedi/slapos!30)

Those customization do not work anymore and it is preventing tests to run,
 so it should be reintroduced later in a way it works.

For upcoming GitLab SR we need Go[1] language support, because one
GitLab service is written in this language:

    https://gitlab.com/gitlab-org/gitlab-workhorse

Here we provide golang component, and helloweb-go service integrated
into helloworld SR.

The patches are based on recent helloworld & helloweb restructuring (see !23).

/reviewed-by @jerome  (on !24)
/cc @kazuhiko, @rafael, @alain.takoudjou, @gabriel, @Camata

[1] http://golang.org