Work around meliae unicode bug

As described here (https://bugs.launchpad.net/meliae/+bug/876810)
simplejson will write out unicode escape sequences, but won't read
them.

This allows the injected process to read the payload regardless of what user
did the injection.

Add support for tox (http://tox.testrun.org/) and Travis CI (http://travis-ci.org/)

add 'urwid' package to setup.py since its a dependency

Interactive interpreter improved.

Using readline if available.
Using a custom code.InteractiveConsole on the injected process side.

Clarify the meaning of the second argument in --help output

This adds a command line option -l (--list-payloads) to list all the
standard payloads available in the 'pyrasite.payloads' package.
It also allows the user to refer to those using just the file name without
a directory, e.g.

    pyrasite $pid dump_stacks.py

It's possible to disambiguate standard payload names from local user files
with the same name by using explicit paths, e.g.

    pyrasite $pid ./dump_stacks.py  # runs user script, not standard payload