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.
I think I will try to make the textures internal to the Ploma plugin as embedded base64 data instead of requiring that the user specify a texture image in the HTML.  I am still trying to decide about this, but I'd like to control which textures are used as they are very specific to the rendering, and it would simplify integration so that only a `<canvas>` is needed in the HTML, and nothing else.  What do you think?