spark - fixes to Selection, Projection, By,
        added ReLabel
Merge branch 'reorg' of github.com:ContinuumIO/blaze into spark_compute
- brought in try/except of pyspark import
- added TODO on selection
Conflicts:
	blaze/compute/spark.py
Merge pull request #318 from talumbau/spark_compute

Spark compute
REMOVE-ME: clear out spark test
add scalar.core
add booleans and numbers
basic columnwise fusion function
more sophisticated columnwise merge test
table exprs depend on and use scalar exprs
add eval_str method to scalars
columnwise doesn't include literals in arguments
make columnwise nary, start removing old python compute code
Python: handle UnaryOp
compute: python ColumnWise and By
compute: move columnwise_funcstr from python to core
compute: Pandas columnwise
compute: redirect ColumnWise compute to expr.BinOp/UnaryOp
revert pandas solution back to dispatch on scalar expr