CPack Packaging Capability

I found myself having to install this on a bunch of machines this week for our school's robotics team, so I wrote some CPack code in to the cmake script.

If you're on an OSX system, it will default to a .tar.bz2 archive, otherwise it will create a debian (aptitude) package.

I also added some header file checking and checking for realpath to make sure that the system we're compiling for actually has the POSIX realpath function.

If you don't want to pull, no biggie, I've just been using this project as an example of using CMake for some of the less experienced members of the robotics team who were interested.