Added google test to facilitate unit testing

To make sure everything is working correctly I added several basic unit tests for merging of basic blocks. I haven't been able to figure out how to get make test to run the test suite and provide meaningful output. However, in the mean time, I added a -test option, so you can run the unit test by running ./cpsl -test. I'll keep working on getting make test to run the test suite and write some more unit tests.