Bugfix. Always use the real document of the checkbox element.

[taueres]

If the checkbox element is in an iframe its document and the main page document are not the same object. In these situations Switchery won't work properly.

[abpetkov]

@taueres Thanks for catching this bug!

Please make sure you generate a standalone version (development) and bump the version with a patch increment on all places it occurs (comments in JS/CSS files, package.json, bower.json, component.json)