git_repository: add download() to clone repo
git_repository: implement methods

* checkout()
* get_commit_id()
* get_refs()
* GitRepository.download()
git_repository: spec: add some tests
base_repository: add dependencies to snippet.json
repository: add to_snippet_json()
repository: spec: add test for is_git_repo()
repository: use read method instead of @var_name
rubygems: use rugged to access git repository
social_snippet: test: use BaseRepository instead of GitRepository
rake: add spec_use_fs task
base_repository: add url property
repository: spec: use BaseRepository instead of GitRepository