Unit tests

This check in adds the JUnit unit test framework to SemGen. We can now
add unit tests. I've created a unit test class for CellMLReader that
contains a default unit test that always passed. Next I plan on writing
a simple unit test to verify reading cell my files works properly
CellML Reader Unit tests

Adding a couple unit tests for the CellML reader. Here's what they do:

1) Verifies the reader can read a file successfully
2) Verifies the pubmed annotation is read properly

There are many many more tests that should be added. This should get us
started.