Add statistics endpoint /api/waterpoints/stats

Returns number of waterpoints of a given status in a given district
Add statistics endpoint /api/waterpoints/stats_by_district

Return number of waterpoints of a given status for each district
Use Eve's send_response for custom endpoints

This returns the proper response format, add adds headers according
to Eve's configuration e.g. CORS.
Add a simple status endpoint.
First set of working d3 plots to start the discussion.
INDEXWIP Creating an UI mockup for the dashboard
Merge branch 'dashboard' of https://github.com/taarifa/TaarifaWaterpoints into dashboard

Conflicts:
	taarifa_waterpoints/taarifa_waterpoints.py