Load HMDA data

Many features have been added to help with loading HMDA data. In particular,

* If a [Drakefile](http://blog.factual.com/introducing-drake-a-kind-of-make-for-data) exists in a dataset directory, it is run. Note that it runs the default task, and in HMDA, you actually do need to go run a second task. This should be fixed in the future.
* Each table is loaded into a MongoDB collection and then slices are built as separate collections. This is in preparation for adding more features to alter slices upon load.
* File loading agents no longer freeze on missing files (!)