Add JUnit tests in src/test

[LeafHacker]

src/test should include unit tests for each public class of clientapi. These should try to test that the API performs as expected and should be runable without minecraft being open.

This should be as simple as creating a test class for each class to be tested and using the JUnit API to test it performs as expected.

A new source set could be created for running the example client, which if set up correctly will probably fix #15