Allow per-repository queueing setting.

This utilizes settings to allow configuring a maximum number of
builds to run for a specific repository.
Ensure repo maximum doesn't override account limit.
Extract a new method, clarify variable names.
Bring back together what once was.
And the award for weirdest Ruby goes to....
Add a config flag to enable/disable limit per repo.
Merge pull request #367 from travis-ci/mm-limit-per-repository

Allow per-repository queueing setting.