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 (`'Connection-ID'` 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.

This results in having to bump to Punchblock 2.0, and I'd like to get a bunch of other changes into that release also:

* [Better Rayo compliance](https://github.com/adhearsion/punchblock/pull/137) (though this will likely not be compliance with Rayo 1.0 since that'll take a while to ratify, and will likely require us to bump to Punchblock 3.0 when that comes around)
* Actor-ification of the interfaces between Adhearsion and Punchblock (partly to address things like [this](https://github.com/adhearsion/punchblock/pull/152#issuecomment-18795580))