* Adding new rasterChart mapdc widget with multi-layer BE-rendering support.
The raster chart is a wrapper covering all 2D backend-rendering widgets.
This currently includes pointmaps and scatterplots. Changes include:
1) new point and poly layer mixins that can be added to rasterChart
2) Adds hit-testing support for all layers, including polys. This includes animations and popups. Requires some new css.
3) fixed bug to ensure pixel radius set to getResultRowForPixel is an int
4) several small changes to crossfilter:
a) each crossfilter object is given a unique id
b) added a getProjectOn() function to groups in order to retrieve the projections in a join/group-by query.
c) added flags to extract the sql from certain dimension/group functions rather than run the sql immediately.
5) Change dc-group-all-mixin to cache the last count by crossfilter id.
* adding multi-layer backend rendering examples for maps and scatterplots
