Created Phaser.KeyCode enumeration / pseudo-type

- Impact: none/foward-migration; fully backwards compatible

- Moves the key code constants in Keyboard to a KeyCode type.
  - Duplicates the KeyCodes in the Keyboard object for backward
    compatiblity.
  - KeyCode properties are listed in documentation

- Updates documentaion to refer to Phaser.KeyCode

- Adds in future 'keycode' expansion capabilities,
  as indicated by the KeyCode documentation, if the constants
  are used in code.

Closes phaserjs#2031

Author: pnstickne

src/input/Key.js

@@ -10,7 +10,7 @@
 * @class Phaser.Key
 * @constructor
 * @param {Phaser.Game} game - Current game instance.
-* @param {number} keycode - The key code this Key is responsible for.
+* @param {integer} keycode - The key code this Key is responsible for. See {@link Phaser.KeyCode}.
 */
 Phaser.Key = function (game, keycode) {