Fix 2048 on desktop browsers

One thing I found out while working on necolas#908 this is that the 2048 game
demo does not work on browsers since it listens for touch events only.
This can be solved by listening to both mouse and touch events.

website/storybook/3-demos/Game2048/Game2048.js

@@ -184,7 +184,7 @@ class Game2048 extends React.Component {
     this.setState({ board: new GameBoard() });
   };
 
-  _handleTouchStart = (event: Object) => {
+  _handleStart = (event: Object) => {
     if (this.state.board.hasWon()) {
       return;
     }
@@ -193,7 +193,7 @@ class Game2048 extends React.Component {
     this.startY = event.nativeEvent.pageY;
   };
 
-  _handleTouchEnd = (event: Object) => {
+  _handleEnd = (event: Object) => {
     if (this.state.board.hasWon()) {
       return;
     }
@@ -220,8 +220,10 @@ class Game2048 extends React.Component {
 
     return (
       <View
-        onTouchEnd={this._handleTouchEnd}
-        onTouchStart={this._handleTouchStart}
+        onMouseDown={this._handleStart}
+        onMouseUp={this._handleEnd}
+        onTouchEnd={this._handleEnd}
+        onTouchStart={this._handleStart}
         style={styles.container}
       >
         <Board>{tiles}</Board>