Added documentation for passing the token as header

The documentation was unclear about how to pass the api private token as
header. Updated the doc to include the nessesary information bits and
added an example using the curl command line client.
describe the project access level

the documentation mentions access_level as required parameter, but fails to list
accepted values.
Update docs to reflect that project names are allowed as ID

The API accepts project names in all places where project IDs are expected. Updated the docs to
reflect that.
update grape to 0.3.1 for ruby 2.0 support.

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.
API repository documentation updated, includes infos to return codes

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.
API: groups documentation updated, functions return different status codes

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.
API: session documentation updated and test added
API: issues documentation and API functions updated

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.
API: fix commits pagination

closes #2300
Merge pull request #3095 from fdiary/update-grape-for-ruby2.0

update grape to 0.3.1 for ruby 2.0 support.
API: merge request documentation updated, added return codes to functions

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.
Merge branch 'master' into fixes/api
Merge branch 'feature/api-documentation-fixes' of github.com:Xylakant/gitlabhq into fixes/api

Conflicts:
	doc/api/projects.md