Basic trees-on-map functional, backed by API

Trees now have a coordinate, which might be useful for validating sightings (i.e. check that the tag number given is near the location given). 

We now have TastyPie serving up feeds for the trees and sightings.

The map view uses the API to get the trees and put them on a map. Different icons for if they have Tag numbers or not. Tagged trees link through to view tree page.