Moves 3rd party jars into a separate folder

It is likely that we will use these libraries across multiple projects,
so it makes sense to put them in a top level folder. Also makes it
simpler for us to find 3rd party libraries, rather than having to hunt
through individual projects.
Adds pi4j library

This looks like a really useful library that Will found for communicating
with I/O pins and serial port on the Pi. Added into the repository so
we can start investigating whether this will allow us to work in Java on
board the balloon.