Add partition by

Aside from adding `partitionby` and tests, this also changes  `__init__.py`'s to pick `identity` from `functoolz` rather than `itertoolz`.  (`identity` is clearly functional at base, IMO).

This was tricky to get right, as per the docstring for `partitionby`.  `itertools.groupby` gets you part of the way there but needs the extra `map(compose(list, second), ..)` to work (if you just `map(second...)` it works in Python 3 but not 2.x).  I actually don't fully understand how  `groupby` differs between Py2 and 3, but the current implementation seems to work well.

Fixes #31.