Make GeoTrellis actor system a singleton.

This commit turns the GeoTrellis akka actor system into a singleton for
purposes of the clustering mechanism.  The actor system is given a single
name ("GeoTrellis") and the tests are changed to no longer create many
servers.  The TestServer class is removed entirely, and the tests are updated
to import from testutils.

In the future, the actor system could be made specific to each server by
having the ServerActor pass its context system into the new server, but that
is not currently implemented.
Change return type of GetHistogram to return FastMapHistogram.

Because GetHistogram was returning Histogram and not FastMapHistogram,
we could not modify the settings of the operation (the thoroughput limit).
Make RasterData serializable.
Update akka version to 2.1.2.
Make RasterType serializable.
Make TiledRasterData classes serializable.
Make Server serializable.
Remove TestServer class.

Instead of TestServer, either use geotrellis.testutil._ or Server.empty(id).
Add LoadUncachedTileSet() operation for loading uncached tiles.
Remove log statements from server actor.

The creation of certain log statements (whether or not they were being logged)
was causing significant performance issues.
Update development remoting experiment to use a cluster.
Remote broken import from tile raster test.
Remove TestServer from tasks.
Merged from joshmarcus/geotrellis/remoting_updates