Stomp 1.1

This pull requests implements the following changes:

* support STOMP 1.1 features
  * vhost support
  * protocol version negotiation
  * message NACK
* implement PHP namespaces
* added high-level OO STOMP API
  * added Connection, Subscription, Message, Transaction, Destination classes
* hide low-level functionality
* require XP >= 5.9.0
* complete overhaul of class structure
* no BC(!)
* added examples, README

In contrary to the branch name, there are no STOMP 1.2 features implemented, yet.