Instrument all ActiveRecord model methods

This works by searching the raw source code for any references to
commonly used ActiveRecord methods. While not bulletproof it saves us
from having to list hundreds of methods by hand. It also ensures that
(most) newly added methods are instrumented automatically.

This _only_ instruments models defined in app/models, should a model
reside somewhere else (e.g. somewhere in lib/) it _won't_ be
instrumented.