Add nice backdrop from subtlepatterns.com
Split test task into test-lib and test-js

This will help to avoid running Mocha every other time.
Improve markup and styles

- Simplify things and make more succinct;
- further decompose CSS;
- disallow resizing textareas horizontally;
- add scrollbar to the preview box and forbid text to overflow;
- use semantic <i> for icons;
- fix colors incompatible with the backdrop.
Add ability to push to and pop from the history of formulas
Persistently store the history between sessions
Add keyboard shortcut for "Push to history" action