Remove keyboard support. Re-factore code.

I removed the keyboard support event binding by default since the
keyboard works even without them (tested on latest build of Chrome,
Safari, Opera and Firefox). To enable this back for some weird reason,
do a {keyboardSupport: true} on initial load.

I refactored the code a bit. I

Other changes:
- moved the `$.extend({},defaults)` to where it's needed (during a
  NanoScroll instantiation)
- Added a removeEvents method call in addEvents.
- Added an visual illustration on how nanoScroller works. (will add
  these later in the README.md files)