API import management command

First pass at adding a command to import via the data.dconc.gov API. There are some discrepancies from the last export, so I'm still working through it. Eventually I think we should reduce the size of our models to only the fields we need, but for now I'll continue to import everything. refs #45 

Schema changes in the dconc.gov API:
* Date formats are now http://en.wikipedia.org/wiki/ISO_8601.
* IDs are all returned as floats, rather than integers.

Current status:

* [x] Establishments
* [x] Inspections -- referenced est_id's don't exist. Emailed Mickey.
* [x] Violations