Add method to delete branches

I have not quite made the spec run correctly
Fix doc. Use delete_ref for delete_branch
Re-write spec to use api-playground/api-sandbox
Correct parameter for updating label name is :name.
Add .update_branch method
Merge pull request #375 from barisbalic/update-labels-documentation

Correct parameter for updating label name is :name.
change test to check login variable instead of hardcoded value
Merge branch 'delete-branch'
Merge pull request #376 from joeyw/fix-gist-spec

change test to check login variable instead of hardcoded value