Add ability to record Rails request ID

~~**Merge after #37 - I branched off that branch to avoid creating conflicts with my own PR**~~ Done

Something I find really useful in Rails is the ability to correlate cross-service requests [using consistent request IDs](http://api.rubyonrails.org/classes/ActionDispatch/RequestId.html). I'd love to have these recorded in query logs as well, so I can correlate DB calls all the way back up to a request hitting the web server at the front.

This PR adds that ability to Marginalia.

cc @arthurnn 
I think it's nil when annotating queries run from outside controllers (e.g. in a Resque job).