Geo-enable datastore/datapusher

Suggested via https://twitter.com/timwis/status/631068707955961861

"Do you have any plans to add geospatial queries to DataStore API? I don't see it on the roadmap and surprised no one's asked"

**Datastore**

A possible starting point (and an assumption) is that if the datastore db is postgis, that `datastore_search_sql` will 'just work'. The other action methods would probably need further work (to ensure insertions etc).

**DataPusher**

The real problem is likely the type-guessing in datapusher, it needs to be able to determine that the float it has just found is a lat, or that the string it has is geojson (for example). Perhaps as a first pass this could be done based on convention of column headers? It's a bit fragile but would simplify a first version.

Possibly related to #151

Start by reading the DataStore API actions, especially datastore_search_sql, alongside DataPusher's type-guessing behavior. The issue leaves the query, insertion, and geospatial type-detection requirements open; done would require an agreed scope and tests covering the selected DataStore and DataPusher behavior.

Written by the indexing model from the issue text.