Make Rayo models plain Ruby objects

Export to XML / Import from XML if connecting to a real Rayo server by XMPP

This includes the following BC breaking changes:
* Headers/AMI attributes were being symbol-ified ('ConnectionID' became :connection_id). We now retain case and use strings.
* AMI events/actions used #attributes(=), which is reserved by Virtus. We now use #headers(=), which is in line with RubyAMI, but retain the old reader to make transition easier.