Consider Implementing Multiprocessing Via Pool

[danielsclint]

The multithreading logic could probably be streamlined and more safely implemented using the multiprocessing.Pool. Right now the code has separate methods for a single thread and multiple threads. This seems like it could lead to fixes being accidentally only applied to one half of the implementation.

The todo_queue seems a little messy, so this might take a little thought and a lot of testing to ensure code doesn't get broken in any fix.

This issue is related to some of the discussion in Issue #89.

https://github.com/BayAreaMetro/fast-trips/blob/241c9f210b71498bad6856a649a1b6c6aa891e82/fasttrips/Assignment.py#L811