Asyncprogress

Rebased and fixed. Replaces #175 and #181. Closes #169.
Because I did not think of it... :/
Please, go ahead and update this branch if you can. You did write almost all of it in the first place. I would like to see this feature in an acceptable state so we can merge it in time for the new version of **NAN**, which I have sort of promised will be out by the end of this month.