Allow free range pasting in HTML mode, but prompt user before toggling views if HTML needs clean up.

- do not parse pasted text in HTML mode
- do not parse text when toggling from composer mode to HTML mode
- when toggling from HTML mode to composer and parsing changes were made, show user changes and give them option to keep or correct.

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Added helper libs and functions:
- diff.js (@jrallison, should prob be in own file)
- getDifferenceString on editor/composer/textarea
- isTextarea on editor

![my image](http://i.imgur.com/ARNKJ.png)