Pass token into the path variable

If a token is already present in the path, the new variable
is ignored.  In order to properly manipulate the path,
a new method, `WebUtil.injectParamIfMissing` was introduced.

Fixes #536

[@directxman12: fix up path manipulation logic]

vnc.html

@@ -199,6 +199,7 @@
                 <li><label><strong>Host: </strong><input id="noVNC_host" /></label></li>
                 <li><label><strong>Port: </strong><input id="noVNC_port" /></label></li>
                 <li><label><strong>Password: </strong><input id="noVNC_password" type="password" /></label></li>
+                <li><label><strong>Token: </strong><input id="noVNC_token"/></label></li>
                 <li><input id="noVNC_connect_button" type="button" value="Connect"></li>
             </ul>
         </div>