Add a mixin for capturing exceptions/messages from a Rails controller

This is the equivalent of Airbrake's [ControllerMethods](https://github.com/airbrake/airbrake/blob/master/lib/airbrake/rails/controller_methods.rb), except simpler.

To get there I had to refactor how Rack exceptions are captured. See the commit message for details.

I also had to depend on RSpec >= 2.10 for the new [yield matchers](https://www.relishapp.com/rspec/rspec-expectations/docs/built-in-matchers/yield-matchers).