Add filterable OTM tile layer

I encapsulated the manipulation of the tile layer URL in a custom subclass
of OpenLayers.Layer.XYZ so that the application can call a simple setFilter
method on the layer when the user changes filter arguments.
Add support for dict values to min/max

Refactor search to be a little cleaner, use a dict of predicate_types
to determine logic instead of if statements.
Merge pull request #63 from steventlamb/topic/refactor_search

Add support for dict values to min/max
Fix instance request helper

Include the instance as both a request property and as an view function
argument for flexibility and to keep "template only" requests simple.
Merge pull request #61 from jwalgran/topic/filter-map

Add filterable OTM tile layer
Merge pull request #65 from jwalgran/topic/fix-instance-request-helper

Fix instance request helper