pyspark addition to compute layer and tests
join operation on pyspark rdds
.travis.yml installs spark
- separate out RDD join
- increase clarity of test cases
Merge pull request #1 from mrocklin/pyspark2

Pyspark2
groupby/join for spark added
wget for travis
handle some spark serialization issues

1.  itertools.tee doesn't serialize well
2.  pre-resolve the dispatched compute function