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.
Ah right. There are some other instances where we share blocks between apps, but not generated code (i.e. the math blocks for calc and eval). The approach I took there (which is not necessarily one I'm super excited about) was to have the block in the shared file, and then override the code generation in the app specific file - see modifyCalcGenerationCode in calc.js.  Feel free to (1) leave as you have things now, (2) convert eval's version to just override the code for fucntional_String or (3) think if you can come up with a better approach than what I've done thus far.