Enable dispatch of operations to a cluster.

This pull request has a wide variety of commits that came up in the process of working through an example of executing a distributed operation through a cluster (associated with the akka cluster functionality).

The changes include:
 -- Update development remoting experiment to use a cluster.
  -- 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.
 -- 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 a variety of classes serializable.	
 -- Update akka version to 2.1.2.	 
 -- 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.