Implemented message deleting


This allows you to delete a message (or call) and makes this code work as expected:

https://github.com/twilio/twilio-php/commit/159960c415a7b296349d3b7e68ef788ca28d24b2#diff-74d2e3a0717d829d8824221b07bd8e5c

Note: This might be better implemented on the Message and Call objects as the delete() doesn't apply to all resources.