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.