Update examples to use onInput instead of onKeyUp

onInput has the advantage that it responds to repeated key events before
onKeyUp and is called when modifying the input without the keyboard
(such as pasting with the mouse).

Test Plan:
Opened the ballmer-peak example and docs homepage in Chrome and checked
that both examples update whenever the text is changed.

_js/examples/markdown.js

@@ -11,14 +11,14 @@ var MarkdownEditor = React.createClass({\n\
   getInitialState: function() {\n\
     return {value: 'Type some *markdown* here!'};\n\
   },\n\
-  handleKeyUp: React.autoBind(function() {\n\
+  handleInput: React.autoBind(function() {\n\
     this.setState({value: this.refs.textarea.getDOMNode().value});\n\
   }),\n\
   render: function() {\n\
     return (\n\
       <div className=\"MarkdownEditor\">\n\
         <h3>Input</h3>\n\
-        <textarea onKeyUp={this.handleKeyUp} ref=\"textarea\">\n\
+        <textarea onInput={this.handleInput} ref=\"textarea\">\n\
           {this.state.value}\n\
         </textarea>\n\
         <h3>Output</h3>\n\