Feature/first unit tests

Here's the beginning of adding unit tests. There is still a good bit of work to do to improve coverage (currently 33.6% coverage by line).

Also, this PR depends on #2.
I see no way that this implementation can return a ``File`` object. 

Testing this method is just part of my process -- I use test coverage reports to identify areas that need to be tested, and even if it seems trivial to test the method, testing it helps to make that report more useful.