Added a helper method to check if required parameters are given in an API call. Can be used
to return a `400 Bad Request` return code if a required attribute is missing.
Code clean up and fixed tests.

Conflicts:
	doc/api/projects.md

The API documentation of merge requests contains info to status codes for all functions.
Required arguments are now checked in the merge requests API functions and a `400 Bad Request` error is
returned if they are not given.

update grape to 0.3.1 for ruby 2.0 support.

closes #2300

The issues documentation is updated with infos to status codes and the deprecated `DELETE` function and
how to close an issue. A few more tests added to check status codes of API functions.

Updates the API documentation of groups with infos to return codes. The function calls
in the groups API have updated documentation and return `400 Bad Request` status code
if a required attribute is missing.

The API documentation of repository is updated and now contains infos to status codes.
Code documentation is also adjusted for `GET /projects/:id/repository/commits` and includes infos to
pagination attributes. Tests are updated.

add grape-entity, that is split from grape 0.3.0.
remove usage of deprecated error_format, that is removed in grape 0.2.3.

Merge branch 'state-machine-stage-2' of https://github.com/Undev/gitlabhq into Undev-state-machine-stage-2

Conflicts:
	app/models/merge_request.rb

Show only ≤16 lines of codes in a discussion in a MR (issue #2860).

Since search_autocomplete_source rendered with raw all human input should be sanitized to prevent XSS

System hooks code & logic refactoring

Remove function 'check_issue_1059_shell_profile_error' from check.rake as it is not needed anymore