fix(DOM, globals, main, taBind): Fix for #1205 Duplicating content inside <p>

 DOM - added taSelection.updateLeftArrowKey() and taSelection.updateRightArrowKey()
       so that we check the selection after the cursor is moved and correct for the
       differences between Firefox and Chrome.
     - added taSelection.getFlattenedDom() that returns the nodes needed to allow
       the updateLeftArrowKey() and updateRightArrowKey() to function.
       Basically this returns a flattened set of Nodes that have text content as well
       as the associated parent Nodes.
     - insertHtml() now allows the insertion of a text with the zero-width space character.
 demo.html - fixed a bug where is had two editors with the same name.
 globals - added '#text' and 'spam' to the VALIDELEMENTS
         - defined ENTER_KEYCODE, TAB_KEYCODE, LEFT_ARROW, RIGHT_ARROW to make the code
           more transparent.
 main - small formatting change to the code.
 taBind - now use ENTER_KEYCODE, TAB_KEYCODE, LEFT_ARROW, RIGHT_ARROW definitions from global
        - shifted to using nodeName whenever we call match(VALIDELEMENTS)
        - element.on('keyup') now detects the LEFT_ARROW and RIGHT_ARROW and calls the
          taSelection.updateLeftArrowKey() or taSelection.updateRightArrowKey()

Author: JoelParke