Ability to check whether a branch exists against Github repo for a job

[Shadowfiend]

When we ask hubot to switch a job to a branch, it would be handy to double-check whether that branch is valid before firing off a build that will fail.

Some thoughts:

• This can be read from Jenkins config and cached in the brain.
• Users who want it re-read would clear the brain (if it becomes a common use case, possibly providing an interface for it would make sense, but not for now).
• Github credentials would be needed.
• This would be optional, so if no github credentials are present, we could skip the validation altogether. Or, if the remote repo isn't in github, same deal.

[riveramj]

7 should add this. Take a glance and let me know what you think.