"hub pr" still untested.

Help still untested.

WARNINGS:
* Entirely untested
* "hub pr" currently works as a synonym for "hub checkout", it should
do something else

The current functionality should really be moved to "hub pr checkout".
This is a step on the way.

It wraps functionality already provided by "hub checkout". The advantage
with "hub review" are twofold:

* Discoverability. It wasn't until I started browsing the hub source
  code that I realized this functionality was present.
* Usability. Just typing the PR number is easier than having to figure
  out its URL.