Add some local event tracking

[jonespm]

It might be possible to do the event tracking all through GA in #239 but if we'd like to do additional event tracking, for instance when a user adds/removes a tool from their course site we may want to have that in a local database. I'd implement #239 first and see if sending a few custom events to GA seems good. Really what we want is to get an idea of usage and which tools are the most popularly added.

In past projects we've used https://github.com/pinax/pinax-eventlog but there may be something better available.