added `add_collection` call to `finance.volume_overlay`

This was raised in http://stackoverflow.com/questions/17251871/adding-volume-overlay-in-matplotlib/17252516#17252516

With out this line, the function updates the axes limits, but
does not actually plot anything.

This is (imo) a bug and should probably get cherry-picked into 1.3 if that is not too late and the 1.2.x branch.