try-except import rather than explicit version check
Merge pull request #317 from mrocklin/spark

Spark
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