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

This PR handles ingest of TSV/CSV/JSON files into Mongo DB collections
Right, but won't this be circular since I defined:

```python
@dispatch(Collection, CSV)
def into(coll, d, if_exists="replace", **kwargs):
```

I suppose I could do something like:
```python
dd_intto_coll = functools.partial(dispatch(coll, type(CSV))
dd_intto_coll(coll,CSV)
```