Add support for creating new branch if it does not already exist.

[thisisaaronland]

• https://developer.github.com/v3/repos/branches/#list-branches
• https://stackoverflow.com/questions/9506181/github-api-create-branch

Not sure what the corresponding code-y bits are, yet:

• https://godoc.org/github.com/google/go-github/github#pkg-examples