File Tracker implementation with polling.

@bsoetaer 
It would be a lot of work to use INotify without using any external libraries, and since Miller hinted in class that polling would be OK for this part, I figured that would be the best way to go.

For part 2, how much are you implementing in C?  I think that a precise sleep method should be enough - then we can reuse it in part 3.
I agree with that, but the only realistic option is to use the ruby-inotify gem, which probably defeats the purpose of the assignment.  So unless getting an external library to do effectively everything for us is permitted, realistic options are out of the question.