add color swatches to layers sidebar, consistify with xray popup

Adds consistent color-coding to the layer drawer and xray style, to tie the two things together conceptually and help users orient themselves:

![screen shot 2014-07-02 at 12 05 36 pm](https://cloud.githubusercontent.com/assets/108094/3458555/bba99c5a-0202-11e4-9111-5e898ba292ff.png)

One concern that had been raised about this is that it could be confusing when xray mode is turned off, leading user to wrongfully think these swatches are coordinated with their carto style.

We could do something like make all swatches gray if xray mode isn't on, but I think that'd be unnecessary.

cc: @nickidlugash @yhahn