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 think I can guess why the two delays conflict with each other - Ruby's threading model uses green threads instead of real system threads, and all of its Thread functions account for that.  `nanosleep