Minor refactoring of filter code + conncted to map

Filter is now passed into the map page context & renders a form we use
for updating the map. The 'observed' field basically checks the key in
the hstore 'observations' field, so based on the bootstrap data, valid
lookups are 'Nitrates', and 'Orthophosphates'. Dates should be
YYYY-MM-DD, but really we need to hook up some kind of calendar widget
here.
use proper named URL
properly report results count
tweaked the way counts are reported
Merge pull request #27 from jimr/map-filter

Map filtering