New levels

Eight of them, to be exact. Some of them are quite hard to get all the treasures in, so that might keep the testers busy for a while :p

All of the platforms in these levels have one of the dimensions specified in #29. (Yes, the boxes in level 7 are four platforms stacked on top of each other.)

In addition to that, a few small changes were made:
* Issue #46 is fixed.
* The number of attempts that the player gets is now read from the level XML file (defaults to 3 if not specified, with a console warning message).
* Renamed the 'length' attribute on the platform to 'height'.

I was going to fix a bug with the ball bounces stat as well, but I suspected that I didn't do it correctly, so that's why there's a commit where I reverted my fix for that. It's so minor that I don't want to potentially break other things at this point.

Oh and here's a screenshot of level 5 :)

![level5](https://f.cloud.github.com/assets/1811645/404514/74079338-a94e-11e2-9628-dcf22ef2ef3c.PNG)