Change the functionality of (json_)api_call

Warning: this diff is difficult to read because some, but not all,
indentation changes affected behavior.

Always set the content-type to application/json, including cases where
the request function returns an HttpResponse.

Drop underlying api_call function entirely and promote json_api_call
to contain its body.
Add view for returning boudaries as geojson
Merge pull request #34 from steventlamb/topic/region_endpoint

Topic/region endpoint
Rename BoundaryZones to Boundary
Merge pull request #35 from steventlamb/topic/rename_boundary_zones

Rename BoundaryZones to Boundary
Add url endpoint for boundary_to_geojson