Add functional blocks to playlab to set background, title, and player/enemy speed.

This is a step toward building a customizable game in Stage 9 of the MSM Course Outline: https://docs.google.com/a/code.org/document/d/1rG5Z0SzmigwDneLIYArI-IF-VJtDKWjciv70-4Iwxio/edit#heading=h.q8l00xoqxfzj

![basic-game-screenshot](https://cloud.githubusercontent.com/assets/8001765/4870393/8ac60e52-6166-11e4-8c81-6af663754470.png)

Ultimately, students will not be exposed to these top-level blocks. They will be hidden from the user, and will call functions which the student will define.
The installString in eval/blocks calls Eval.string, which does the "Eval.lastEvalObject = object;" hack, which I don't fully understand. If that's no longer needed and I can safely call this installString from Eval, then I'll make the change. Or I can leave this hear and when you remove the lastEvalObject hack you can then change it to point here.