Rename fork_count repo attribute to forks_count

The API response has forks_count, as can be seen in the API doc at
https://developer.github.com/v3/repos/#response
and the fake data at
https://github.com/sigmavirus24/github3.py/blob/4daa290/tests/unit/test_repos_repo.py#L32

The previous attribute was kept for backward compatibility, if desired.
Add self to AUTHORS
Merge remote-tracking branch 'origin/pr/264' into 1.0-alpha
Relocate Repository#is_collaborator
Rename Repository#iter_forks to Repository#forks