Write client and server unit tests

[masomel]

The past release removed a lot of functionality that made automated testing easier. The only way to test the system is by running the server and client directly since they can only communicate over the network.
#16 made interactive testing easy, but removed the ability to test the client automatically.

[masomel]

Should write a test suite testable with Maven, and then display with Coveralls.

[masomel]

Crypto unit tests added as part of #36.