Selector documentation improvement

[pnorman]

https://www.mapbox.com/tilemill/docs/guides/selectors/ does not document all selectors possible

• Nulls ([field!='']), see null not equal empty string (back compatibility issue) mapnik/mapnik#1859
• Booleans

It would probably be useful for the selectors reference to be in with the rest of the cartocss reference, so the language specification is in one place, and multiple versions can be supported.

The document makes reference to numeric values. Numeric is its own type postgres, just like integer and float. As shapefiles also frequently contain numbers in string columns, it would be best to be explicit about what a "numeric column" is.