Added last-status command to fetch last status for given SHA from Status API

I am working on integrating Hub to our Capistrano recipes and I want to abort a deploy when the SHA being deployed is not tested by CI.

Therefore, I added a "last-status" command that displays the single status word "success", "failure" or "pending".

I tried to come up with a bit of better naming to not confuse things with "git status" like:

* ref-status
* statuses
* commit-status

It also needs a bit more sanity checking for when things go wrong.

But, is this a command that would fit in hub?