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.
Nit: the trailing underscore is meant to imply that this is a pseudo-private method, and we should be calling it from outside the class. It looks like we already violate that in a number of places in blockly-core. Perhaps we should just call it quote. Also, the comment in that method declares it to be a hack and should be replaced with goog.string.quote. Is that something we should do if we're going to start using this more widely?