Add ParseContext hook for adding exec globals

Hey John, as I mentioned, I didn't see an obvious way to put foursquare-specific globals in the exec context of ParseContext, so I added this hook.

I am using it locally in our top-level BUILD.bootstrap file like so:

  ParseContext.add_to_exec_context('from foursquare.pants import scala_record_library')

to save us having to type that import at the top of 100s of BUILD files.

Let me know if this looks OK to you or if there is an easier way that I've missed to get the same result.

Thanks!