Moved rendering functionality in ploma.js to have it available as a plugin

created Ploma module object in ploma.js and moved initialization to init.js. Ploma encapsulates Sketcher prototypes - to create an object that renders stroke arrays, create a Ploma.Sketcher instance with parameters canvas, textureCanvas1 and textureCanvas2. A sketcher instance provides and interface with addPoint, beginStroke and finishStroke and has public values strokes (Array) and curStroke, both the filtered version of points/strokes.
@athomschke 

Thanks for these changes!  I think they are a great start to turn Ploma into a reusable module, especially for multiple instances on a page.

I would like to make a few changes to the overall structure of your changes before merging the pull request.  I will leave a few of the changes as questions throughout this PR so you can please let me know what you think, especially if you think they will still work for the integration with Lively.  Thank you!