[WIP] MongoDB coll <- TSV/CSV/JSON 

This PR handles ingest of TSV/CSV/JSON files into Mongo DB collections
so far, we've been able to do tests with smaller files we create on the fly with `tempfile`. I really like that style. We also need testing with other datasets, as discussed in https://github.com/ContinuumIO/blaze/issues/217. Is it possible to do a test here with `tempfile` for now?