Fix SerializationSuite failures due to SharkContext NPE

Changes:
- SharkEnv always contains a reference to SharkContext (as opposed to SparkContext)
- initialize() methods to Shark's KryoSerializer singleton, to make it easier to pass SparkConf references needed for SparkKryoSerializer

- always launch with Java for `bin/shark-shell`, due to scala-2.10's `scala` command loading a conflicting version of Akka.