Feature/crossfilter

depends on #605

This is the start of the crossfilter widget.  

On the left, you have a  list of all the columns, and some information about them (for continuous data, quantiles, for discrete factor data, some indication of the number of unique values).
You can drag them into a filter area, where you can choose how you want to filter a data (so for discrete factors, you might have a select box where you can decide which values you want to look at, for continuous factors, you would have a histogram of the data, and you could select ranges you want to filter by). 

After that filtering, you would have a UI for constructing plots - deciding which columns to map to X and Y, and what type of glyph to use (bar/line/scatter with marks, etc...)  You would also be able to choose how to facet (so facetting is like making a grid plot, so if for example with the cars mpg dataset, I might facet by make and year, in which case I would get a grid plot, where the columsn would be the different manufacturers (toyota/chevy/etc..) and the rows are years (plots for 77,78,79....etc..)