Add plot popup view and template

Generates the content for the popup shown on the map when the user
clicks a tree.

I named the view that routes to the tree detail page so that I could
easily refer to it in the plot popup template tag.

I did not spend a lot of time adding data-* attributes or style
to the markup because we are waiting for full HTML prototypes from
the designers.
Show popup when clicking plot on map

A little more complicated then I would like, but the UTFGrid
control only returns the interactivty properties for a feature
and you need both data and coordinates to show a popup. My
solution uses Bacon to zip a stream of popup content with a
stream of map click coordinates.

I refactored the existing UTF grid hover code to use the new
asEventStream method whic I added to OpenLayers.Control.UTFGrid.

The popup has a link to the plot detail page. I added the
plot ID to the plot detail page template to make the change more
visible in demos.
Merge branch 'topic/plot-popup'

Conflicts:
	opentreemap/treemap/urls.py
Remove lint from treemap/tests/models.py