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.
we could use goog.string.quote... but that's part of Google Closure whereas I think we decided to favor jQuery within blockly-apps code. This code is super closely tied to blockly-core though so maybe its ok. proposal: add blockly.JavaScript.quote (no underscore) which calls goog.string.quote? that way the dependency chain stays clean, which only blockly-core depending on Closure.