change the repository clone_at option to checkout_branch to match
libgit2 and assign it. add tests for checkout_branch
Improved code based on comments from @arthurschreiber
Remove the test with ssh credentials and branch since it breaks the travis build. Added assertion to test_clone_with_branch
Merge pull request #339 from albertsun/repository-clone-at-checkout-branch

Rugged::Repository.clone_at checkout_branch option