- Replaced modified Kryo version with the standard one augmented with
the kryo-serializers package, which includes support for classes with
no-arg constructors (that was why we had a modified Kryo before)
- The kryo-serializers version also fixes issue #72.
- Added a bunch of tests.
- Serialize maps and a few other common types properly by default.
thus clean up the various singletons (RDDCache, MapOutputTracker, etc).
This isn't perfect yet (ideally we shouldn't use singleton objects at
all) but we can fix that later.