Renamed from mavlink* to mavbridge* to disambiguate.

Reworked command-line options; use '-h' for details.

Major gutting and refactoring to support extensibility:
* mavbridge_core.py contains a core class that blends rospy and pymavlink together and provides a very thin wrapper
* mavbridge.py provides a few "standard" ROS publishers and subscribers, and the command-line logic
* mavbridge_*.py are modules that can be loaded in with the '-m' option