Add one from jaraco.util.itertools 9.0
Polish up one() for merge.

* Move a one() corner case from a doctest to a unit test. No need to clutter the docs with that.
* Rename the formal param for consistency with other routines.
* Make the doctests pass in both Python 2.x and 3.x.
* Be more specific about what exception one() raises.
* Slide one() up in the docs so the routines are in alphabetical order. Soon, I'd like to break them into categories so people can skip reading ones they don't need.
Merge in one() from jaraco.util.itertools. Close #33.