Add option to leave DOM in place when destroy() is called, add proper cleanup of generated_stylesheets.

Add the option suggested in #218 to allow clean destroy by running destroy(false). This allows you to use gridster  on an element, and then destroy gridster without destroying the original DOM elements. This is what I'm using to cleanly implement window resizing at the moment.
* Add some code to properly strip out generated style tags when destroying the gridster instance. It also removes the entries in the Gridster.generated_stylesheets static var. This should allow multiple gridster instances to play more nicely on the same page. This should take care of #284, #227, and is a more complete solution than #287.