input.js: uncomment special key handling for non-US keys.

Issue #21 - non-US keyboard layouts.

The code section for tab, backspace and enter was commented out for
testing but got checked in that way. Fix that.

include/input.js

@@ -45,7 +45,6 @@ that.set_target = function () { throw("target cannot be changed"); }
 function getKeysymSpecial(evt) {
     var keysym = null;
 
-    /*
     switch ( evt.keyCode ) {
         // These generate a keyDown and keyPress in Firefox and Opera
         case 8         : keysym = 0xFF08; break; // BACKSPACE
@@ -55,7 +54,6 @@ function getKeysymSpecial(evt) {
         case 9         : keysym = 0xFF09; break; // TAB
         default        :                  break;
     }
-    */
 
     if (evt.type === 'keydown') {
         switch ( evt.keyCode ) {