Simplify SQL & GeoJSON logic by using a RealDictCursor and simplejson

The SQL now does not contain any formatting specific to JSON apart from
the geometry. The RealDictCursor returns the rows as a plain dict and
formatting values that can't be natively encoded in JSON is done via the
default keyword arg to simplejson.
Remove superfluous contextlib usage
Add details of JSONP support
Merge branch 'master' of https://github.com/AstunTechnology/planning-hub