Add parallax support (for issue #225)

0 means parallax disabled

Otherwise, the value corresponds to a scaling value. 1 is doesn't really
scale the layer. Values greater than 1 will zoom in to the layer and values
between 0 and 1 will scale down the layer.

Some adjustments may still be necessary.

The pull request has been done on the master branch because the dev branch was too foreign (with the javascript editor). The changes should be able to be ported to the dev branch.