* If you load the squire.js script into a top-level page rather than an iframe,
it will add a Squire constructor to the global scope.
* The Squire constructor can be used to instantiate multiple instances on the
same page without having to load/parse/execute the full code every time.
* For each instance, create a new iframe, then call `new Squire( document )`,
with the document node for each iframe.
Firefox incorrectly goes back/forward in history instead of moving the cursor to
the beginning/end of the line when you press cmd-left/right on a mac. We now
override this to do the right thing.
