Support for location search for samples

The presence of a query parameter 'location__contained' in the request with bounding box coordinates will now append an ORM filter to search for samples which fall within that bounding box.

Bounding boxes usually have the format: top_lat, top_long, bot_lat, bot_long

Example: location__contained=71.441048,-66.940643,18.911720,-179.150558
Merge pull request #6 from kristallizer/api

Support for location search for samples