Draw strums on in the svg element in stead of on a canvas.

This means that strums now can be easily styled using plain css.
Rename the pinch canvas layer to strum and style it in the code.

I had hoped of getting rid of the layer altogether, however I need to
"install" the drag behavior on it. I tried to install it on the svg element
without luck and installing it on g#strums doesn't work either because it's
size is based on its content, so it won't "see" any drag events.

The styling of the strums canvas, which enables listening to mouse-vents,
must be done in the code (as opposed to the css) for reasons mentioned in
pinchable.js.
Rename pinchable.js to strumable.js
Rename the example file for strums to strum.html.
Get rid of the last pinch references.
Update the documentation in the strum example.