webapp: add canvas APIs

![screen shot 2014-11-17 at 1 05 40 pm](https://cloud.githubusercontent.com/assets/5429146/5077877/75f3f50e-6e5c-11e4-91b1-5f3663447d7b.png)

* Added a few canvas related APIs to webapp: createCanvas, canvasDrawLine, canvasDrawCircle, and canvasClear (clearly, more will be needed later)
* canvas elements for now are always 400x400, filling the entire visible area. the z-index of the divWebapp and canvas elements have been adjusted to make event bubbling work while elements are positioned overlapping with the canvas, while maintaining visibility of the canvas drawing.
* tweaked the webapp interpreter loop to match the recent changes in studio (improves speed)