Demo Changes

These changes are result of driving geotrellis-spark on a cluster to provide demos for presentations. They include some optimizations and a lot of bug fixes.

Most notably geotrellis-spark now does not have a hard dependency on spark and user must use spark-submit to launch it. This makes it a lot easier for geotrellis-spark to be used a library as spark-submit takes care of shipping JARs around.

Also note that Catalog now saves the raster histogram. This allows us to pain each tile with globally appropriate colors.
Abstractly I think of RDD as a table of K,V pairs that is split. I'm not partial to the name though. Mostly I wanted to have mapKeys and mapTiles as separate functions so the code is easier to read. Of the two you suggested I like 'mapPairs' best. I'll make a commit for that rename.