fix for depending on css context as same than javascript context

Added a new option "document". The current version 2.0.1 has a problem, where loading the spin.js javascript code from another frame, added the css rules to that other frame. Basically it came down to wrongly relying on the "document" where the javascript is running as the default setting.

I added this for use of spin.js in a chrome extension, but I'm sure this bug also happens when using iframes, multiple frames, or other stuff like that.