Add variable inputs to Play Lab actor blocks

Allow play lab blocks to accept a variable for actor number.  The existing studio/blocks.js code is a little messy.  I tried to follow the existing conventions for the added blocks, but we may want to refactor this file in the future.

<img src="https://cloud.githubusercontent.com/assets/413693/4959508/dde5b00e-66b8-11e4-9d61-b585980b937f.png" width="500">

I'd like to avoid this opt_* pattern in blockly if we can. I prefer just always requiring all params, or a second best option of checking if the param is undefined and setting it to a default value of false on the first few lines of the function.