add fold

Fold is an associative version of reduce that

Requires an associative operator
Supports parallelism
add reduce to compatibility
propagate chunksize to recursive fold call
fold supports no-default and is mostly lazy
Replace coll with seq in fold
fold uses ``combine`` to merge intermediate results
move fold from functoolz.core to sandbox.parallel
remove fold from functoolz
Merge pull request #116 from mrocklin/fold

add fold