* Add timeoutInterval property on RKRequest with a default value of 120.0
* Add timeout method that is called by the timer when the timeout interval has been exceeded to cancel the request and return an error via didFailLoadWithError:
* Add invalidateTimeoutTimer method that is called by RKResponse when the NSURLConnection begins receiving data.
* Add call to invalidateTimeoutTimer in RKRequest cancelAndInformDelegate: so we don't have a dangling timer.
* Add timer creation to sendAsynchronously and sendSynchronously