add speed slider and pause

![screen shot 2014-10-22 at 5 04 12 pm](https://cloud.githubusercontent.com/assets/5429146/4746390/f6001ffc-5a48-11e4-8626-722ca0da5e2a.png)

Added these new items to control interpreter speed and pause/continue execution.

Also, tweaked the speed of the interpreter to go faster when not running "user code"