added a new option to enable/disable keyboard navigation
- Added websites in "who is using fullPage.js"
- renamed option ‘useKeyboard’ to ‘keyboardScrolling’;
- set ‘keyboardScrolling’ default value to true;
- merged two ifs into a single one;
Merge pull request #196 from andreibosco/master

Added a new option `keyboardScrolling` to define whether or not to allow scrolling by using the keyboard.
- Added website in "who is using fullPage.js"
Merge