fix(destroy): Added bool param to destroy method to leave original DOM in place

Added a new parameter 'removeDOM' to the destroy method - when false,
destroy leaves the DOM in place, but just removes all the gridster
business from it and removes classes and stylesheets it added.

Closes #218
fix(destroy): Added better cleanup for style tag cache when destroy() is called.

The static stylesheet cache generated_stylesheets is now cleaned up
properly when a gridster instance is destroyed. Adds 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.

Closes #227, #284, #287