ENH: Add iterative_batch_transform decorator

Improves performmance of batch transforms functions that could be
written iteratively by passing the previous return of the transform
function as the second argument to the transform function.

* Fixes #115
BUG: Preseve docstring in batch functions

The batch_transform decorator wipes out the doc string of the function
it wraps. Decorate the creator with functools.wraps to preserve function
metadata.