Adding support for the SendGrid event notification API

Hi,

I'd like to add support for the [SendGrid Event Notification Webhook](http://sendgrid.com/docs/API_Reference/Webhooks/event.html). This feature is probably beyond the scope of this project which is why there are yet no tests, and no dependency additions. I wanted to put the idea forward with the basic code to discuss before I continued. Otherwise I can move this into a separate gem. It seems a (fairly) natural fit here, though.

This adds an email event processing controller, which uses `yajl` to parse the incoming JSON body SendGrid sends (I'm using Yajl as it supports parsing streams, and the content received from SendGrid event notifications with batches enabled unfortunately is not valid JSON, it's newline separated JSON objects, using Yajl means it'll work for both batches and single JSON objects).

Would these additions be considered?