Listen to notifications of added/removed stylesheets

Add missing handlers for Stylesheet.Added and Stylesheet.Removed notifications. Add minor changes to ```LiveDevelopment._handleRelatedDocumentDeleted``` to make it work with the current notification format.

Additionally, this PR includes a modification that makes ```LiveDevProtocol``` to use  ```method``` property as the event type rather than always using "event" when triggering the received messages. It allows to develop and subscribe specific handlers per event type instead of just having  a unique ```onMessage``` handler for all the messages.

This PR is on top of #5