filter testing app

@jdeolive, first cut at this. Some filters are hard-coded to demonstrate specific functionality. For the spatial filters, I found it hard to get quality examples and thought it would be more fun/enlightening to allow user-entered geometries. So for the spatial filters, select a tool and draw a feature. Then change the spatial filter and see what happens.

I took the other examples straight from the geoserver page. Two don't work:

1. UNEMPLOY / (EMPLOYED + UNEMPLOY) > 0.07
1. IN ('states.1', 'states.12')

For (1), looks like math is totally not implemented.
For (2), IIRC, geojson yields no id for features

Obviously it would be hard to test 'equals' without having the example geometry but I figured one approach would be to use the `features` endpoint to get it.

Thoughts?