Add plot popup

###  Backend

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.

### Frontend

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.