Improve i_batch

Add grouper from the example on itertools docs

Add batcher and i_batcher
 based on grouper while filtering out the fill value
Add i_batch tests
Reverse batcher arg order.

With two positional args, having the iterable second
supports more cases to use it in conjuction with
functools.partial.
Merge branch 'i_batch' into batcher

Conflicts:
	karld/iter_utils.py