wip: repository: small changes
wip: rake: add spec_use_fs task
wip: repository: spec/test: use BaseRepository instead of GitRepository
wip: rubygems: use rugged to access git repository
wip: git_repository: implement methods

* checkout()
* get_commit_id()
* get_refs()
* GitRepository.download()
wip: git_repository: spec: add some tests