C++ rosserial_server

Working, with the following limitations:

* Functions standalone, but for full compatibility (eg, with rosbag), requires the message_info_service.py helper node to be running (see launch/socket.launch)
* No support (yet) for logging, parameters, or services. This won't be a priority for me/CPR, as we've typically just stuck to publish and subscribe.
* No tests at this point.

This implementation alternates between the VER1 and VER2 connection strings; it supports any groovy or hydro client. Socket and serial nodes are provided.

This PR addresses #37. Please let me know your thoughts.