Refactored out the agg() and comp() methods from Pregel.run.

Defined an implicit conversion to allow applications that don't use
aggregators to avoid including a null argument for the result of the
aggregator in the compute function.

Note: This test suite currently fails for the same reason that the
Spark Core test suite fails: Spark currently seems to have a bug where
any test after the first one fails.

Configuration in Hadoop 0.20.0