[Finishes #79563856] Add logic (if/else) and parameterized functions to Artist levelbuilder

Make it possible to create more complex logic in Artist levelbuilder — like http://studio.code.org/s/1/level/108 — without needing to hand-edit levels.js.

![screen shot 2014-09-25 at 3 33 31 pm](https://cloud.githubusercontent.com/assets/413693/4413878/efe85aae-450c-11e4-82c2-c696c941963b.png)
any reason you've done this differently than the other checkboxes? i.e. "= boolean_check_box f, :skip_instructions_popup"