Add buildpack information to app usage event.

This adds information about the buildpack used to stage an application to the app usage events.  This intended to be part of the resolution to cloudfoundry/cloud_controller_ng#178.

I added a new column to the apps table to hold the detected buildpack guid but didn't expose it to the api
layer pending further discussions.  I also noticed some inconsistencies in the app usage event.  In some cases the name and guid are added to the record while in others it's only the guid.  I can't tell which is preferred.

There are some rough spots here so, as always, this is a proposal; I'm happy to work out the details.

/cc @markkropf @fraenkel