On paste, ignore image if text/html present.

If you copy a portion of text from word, it includes an image version of the text as well as an HTML version. On paste, we now ignore the image
representation on the clipboard if an HTML representation is present, so that
the text pastes as expected, rather than as an image.
Turn Range prototype extensions into functions.

A step towards being able to share code between multiple concurrent instances of
the editor. Also reduces minified size slightly.
Make into a JS class for multiple instantiation.

* 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.