R1:7ffea0463e42

Use herald to trigger builds of revisions and commits.

Summary:
Depends on D7500.

This seemed like a pretty good idea once I thought of it. Instead of having some custom triggering logic instead Harbormaster, I figured it best to leverage all of Herald's power so that users can create rules to apply builds to commits and differential revisions. This gives the added advantage that they can trigger off builds for particular types of revisions and commits, which seems like it could be really useful (e.g.