Support for bind/unbind exchange

While experimenting with exchange-exchange binding I noticed that the XML spec in the current version already contains support for this feature. I've added the bind_exchange and unbind_exchange methods and a rudimentary test. Also included is a Travis CI configuration file; you can see the test results at https://travis-ci.org/wtsi-npg/AnyEvent-RabbitMQ.

Sorry for the bit of whitespace noise where I auto-trimmed trailing spaces.