Add unit tests

[armadsen]

Some unit tests for packet parsing (Issue #55) exist now. Many tests of ORSSerialPort will require actual hardware. My current plan is to write a testbed app with which unit test code can communicate (e.g. over a null modem cable). This makes it so a (fairly minimal) hardware setup is required to run the full suite of tests, but I can't really come up with a thorough test scheme that avoids that.

Where possible, standalone tests are preferred (as in the current packet parsing tests).