Switch from the shortcut.js to the jquery.hotkeys.js library.

The original shortcut.js library has trouble handling special keys.
For example, if a shortcut for 't' is defined, pressing F5 will
trigger it because the keycode for F5 is equal to the charcode for
't' (116).

See for example branch ```throw_shortcuts_shortcut.js``` in [git://github.com/Sigill/selfoss.git](git://github.com/Sigill/selfoss.git).

Switching to the jquery.hotkeys.js seemed to me a better option than patching the shortcut.js library.