search with optional bbox

[hkrishna]

[DO NOT MERGE YET] - depends on geopipes/elasticsearch-backend#3
initial commit + tests

[missinglink]

LGTM, ditto on geopipes/elasticsearch-backend#3 (comment)

If we enable this by default then bbox will be required for all searches, do we want to do that or fall back to the 50km radius search when bbox is not available?

[hkrishna]

I personally like the idea of it being optional. I can think of use-cases where you want to search within a bbox and when you want to find places closest to a lat/lon.

So, doing a 50km radius search if no bbox is passed in makes perfect sense to me - we just need to document it well.