Fix for working with polymer

Changes:
* `jQuery.UIkit.$html` property added.
* Subscription for events like `UI.$doc.on('event', 'element', func)` and corresponding `.off()` changed to `UI.$html.on('event', 'element', func)` in order to fix incompatibilities with [Polymer Platform](https://github.com/Polymer/platform).
* Removed unused `winwidth` variable in `offcanvas.js`.
* Removed unused `eleheight` variable in `smooth-scroll.js`.

Breaks nothing, but allows work with [Polymer](https://github.com/Polymer/polymer).
The only incompatibility I found was that events subscription like `UI.$doc.on('event', 'element', func)` didn't work. I've changed them to `UI.$html.on('event', 'element', func)` and didn't forget about `.off()` calls.
Also I found 2 completely unused variables, and added `jQuery.UIkit.$html` as it was used in many places just like `jQuery.UIkit.$doc`

I'm already using this version, it works fine, you can found js build here: https://github.com/nazar-pc/CleverStyle-CMS/blob/master/includes/js/ab.uikit-2.9.0-git-hacked-for-polymer.min.js