CMake Helpers

This PR resurrects the old rosbuild rosserial functionality of being able to build a firmware right inside your ROS package. Example package you can build with this change:

https://github.com/mikepurvis/hello_arduino

Some basic wiki documentation will be created for the three CMake functions, as well as filling out the catkin side of [this tutorial](http://wiki.ros.org/rosserial_arduino/Tutorials/CMake).