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.
Doesn't feel like this belongs in a util file, as really it's just a block of it's own. maybe stick in mathblocks.js (perhaps also renaming to something like sharedFunctionalBlocks)? if we'll never need the operation based blocks in studio, we could also try to be smarter about which of the shared functional blocks we use (if we care).