Add module for exporting the data back out to csv

I've added a module to export the data back out to csv. It's not quite complete but I think I'll need some help with the final piece. I've got it looping through each GTFS table and creating the appropriate .txt file, but I can't figure out how to get the table headers and rows written to the csv file. It seems like it should be easy to do and I've been reading through the SQLAlchemy docs, but I'm just not getting it. Do you mind taking a look?

You can ignore the pull request for now. I can re-send once I have it all working properly.

P.S. I'm pretty new to all of this so if there's a better way to have you review what I'm doing, please suggest.