Initial commit of maki icon set.

This is a separate pull request just to make the other I'm about to open easier to review.

The Maki icon set is used in the GeoJSON simplestyle specification and will also provide a good default icon set for use with the new `PinBuilder` class.  The icons weren't usable out of the box, so they had to be massaged in a process that I've detailed here: 
https://github.com/AnalyticalGraphicsInc/cesium/wiki/Maki-Icon-Generation for details.