Add unit testing using Google Test

This pull request adds unit testing using the Google Test framework, which must be installed in order to run tests.  Please target version 1.5 of the framework for now, even if you have a later version installed.  It will work, just don't use brand-new features.

Fedora:
    sudo yum install gtest gtest-devel
Ubuntu:
    sudo apt-get install libgtest-dev

Typing "make test" will build the tester and run the tests.