Make paste button visible.

This change makes remix work again after changes gh-book side. We no longer
hide the paste button, we merely disable it in the plugin.
Make math popover a little wider, to make space for cheatsheet and Copy buttons.
Add a bit more space next to cheat sheet checkbox.
Add small css class to make the paste button flash, like it does for gh-book.
Fix for webkit detection and jquery 1.8.

Same as the one in upstream aloha.
Merge pull request #102 from oerpub/remix-fixes-20131024

Fixes to play nicer with remix again