Added branch selection when deploying, closes #26

[reicruz]

You can now specify a branch when deploying a project:

• Requests branches from repo and lists them for the user to pick
• If only one branch exists, automatically deploy it
• If error with request (rate limit reached) check for user provided HUBOT_GITHUB_TOKEN
• If token not provided or another error with the request, user is prompted to manually enter branch

Note: Tests are updated to cover the changes but not using the new testing guidelines established in CONTRIBUTING.md. Currently working on a different PR for updating the tests to use portend. #24

@aeweidne assigned to you since you initially worked on this module.

[aeweidne]

lgtm