Add groups api

Created by: simonswine

Hey,

i reworked my groups api addon pull request (#2523), so that it now contains tests+docs. It also distinguishes between admins and normal users

Cheers

doc/api/README.md

@@ -32,6 +32,7 @@ When listing resources you can pass the following parameters:
 + [Users](https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/users.md)
 + [Session](https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/session.md)
 + [Projects](https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/projects.md)
++ [Groups](https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/groups.md)
 + [Snippets](https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/snippets.md)
 + [Repositories](https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/repositories.md)
 + [Issues](https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/issues.md)