implemented boost asio integration

This is an example of how boost could be used with this library. The example runs and completes correctly

In the main.cpp file a define SSL_DEMO can be set, which makes the demo use an ssl connection. You do need the ca.pem, cert.pem and key.pem files and the port is assumed to be 5671. You also need to adjust the Makefile to link the openssl libs.
For normal connections the port is assumed to be 5672.

I decided to use class interfaces for callbacks, to keep in line with this library. I personally would have preferred a combination of std::bind and std::function for the callbacks