Packet, RadioConnection classes

Wrote Packet abstract base class for all packet types. Wrote a PacketMotion class for motion packets. Wrote/implemented RadioConnection class that handles sending and receiving packets over the radio.

Made test_radioconnection.cpp to test the above classes. They compile; haven't yet tested for logic correctness.

Also renamed LinuxRadio class to RadioLinux, and renamed files to radiolinux.h/radiolinux.cpp, because it made sense to have the file name match the class name.