Added option to override the radius of the effect

useful if you don't want the pow effect to reach to the exent of its
container
Added bgType parameter to indicate the kind of background gradient desired

I used this to create a background with a vertical gradient on
cloudsponge.com
Returning some value to indicate success

I'm returning the element in the case of success, but undefined will
still be returned if no canvas is created and the effect fails.