Fixed empty slots handling (#220); Fixed scaled slots handling (#205)

I closed the previous pull request. This time I did it in a dev branch. 

Just a few comments:
The main point was to do every slot a DisplayObjectContainer and not a Sprite as it was before due to the facts that the image attached to the slot can be already transformed (scaled/rotated/etc) and all the slot transformations should be applied to attachment which is already transformed. It also helps to keep correct draw order in the situation when some slot is empty in the beginning and the image in it appears during the animation.

I also think that it's better to keep the original code of spine-runtimes from esotericsoftware as close to original as it's possible - that's why I removed some changes from it made by PIXI developers (maybe I don't understand something bu it works fine). Now it differs from the original only in the part of attachment creation since we use PIXI loader instead of the the one provided by esotericsoftware.