This commit adds a new graphx-shell which is essentially the same as
the spark shell but with GraphX packages automatically imported
and with Kryo serialization enabled for GraphX types.

In addition the graphx-shell has a nifty new logo.

To make these changes minimally invasive in the SparkILoop.scala
I added some additional environment variables:

   SPARK_BANNER_TEXT: If set this string is displayed instead
   of the spark logo

   SPARK_SHELL_INIT_BLOCK: if set this expression is evaluated in the
   spark shell after the spark context is created.
Merge branch 'master' of https://github.com/amplab/graphx into graphxshell
Changing Pregel API to operate directly on edge triplets in SendMessage rather than (Vid, EdgeTriplet) pairs.
Merge pull request #56 from jegonzal/PregelAPIChanges

Changing Pregel API to use mapReduceTriplets instead of aggregateNeighbors
Merge pull request #49 from jegonzal/graphxshell

GraphX Console with Logo Text
Use custom serializer for aggregation messages when the data type is int/double.
Merge branch 'master' into rxin

Conflicts:
	graph/src/main/scala/org/apache/spark/graph/impl/GraphImpl.scala
Made more specialized messages.
Additional optimizations
merged and debugged
Fixing tests
Adding unit tests to reproduce error.
Merge pull request #58 from jegonzal/KryoMessages

Kryo messages
Fix the hanging bug.
Merge pull request #60 from amplab/rxin

Looks good to me.