Added experimental filter section.

I added the experimental folder to show some work on filters that
can be applied to an entire layer. Multiple filters can be applied
to a layer (in any order, multiple times). To hook into the layer I
use:

    layer.on('draw', filterFunc);

Eventually, I would like to move that to `layer.filterFunc` and
automatically apply it after the draw. `filterFunc` looks like:

function filterFunc(){
  // Get pixel data and create a temporary pixel buffer for working
  var imageData = this.getContext().getImageData(0,0,this.getCanvas().width,this.getCanvas().height);
  var scratchData = this.getContext().createImageData(imageData);
  // Apply all filters here
  ColorStretch(imageData,scratchData,{});
  // Copy the pixel data back
  this.getContext().putImageData(scratchData,0,0);
}

`ColorStretch` is an example of a filter. It takes 3 arguments: the
original image data, image data to write the result to, and an options
object.