Add tests for filesystem walker

[keeferrourke]

This should be able to done by mocking out a filesystem using an osFS{}.

Details for mocking out a filesystem can found in the 10 things you (probably) don't know about Go talk from 2012.

There may be a nicer way of doing this using a package like vfs but I would rather not add another package dependency for testing (I think).

This issue is a more specific discussion from issue #3.