Add API for HTTP request and response events

This is going to require the WebDriver implementation to hook up to a proxy in order to implement (such as BrowserMob).

For PhantomJS, it can already do these things, but the GhostDriver API doesn't have access to these (since it just implements the WebDriver API), AFAIK. Perhaps a fork of the GhostDriver project could add access to these API's if they don't exist.
