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
Fix typo in Repository JSON key 'forks_count'

It should be 'forks_count' instead of 'fork_count' according to https://developer.github.com/v3/repos/#get.
Merge remote-tracking branch 'origin/pr/264' into develop
Merge remote-tracking branch 'origin/pr/268' into develop

Conflicts:
	github3/repos/repo.py