Create rosdistro scheduler

[mikeferguson]

A buildbot gitpoller will work for detecting changes in the rosdistro repository.

At that point, our scheduler should detect that the rosdistro is the change source, find out what repositories have new releases, update the release_version of each of them, and trigger a rebuild.

Some implementation notes:

• Each scheduler has a .master attribute
• The master has a .config attribute
• The .config.builders is our list of builders, already converted from a list, to a dictionary, with with the builder name as the key.

This is blocked on #2