Try not to break regular typeahead when monkey-patching it

This monkey-patches `$.fn.typeahead.Constructor.prototype.render`
in a way that makes it proxy:
 - to the original typeahead render function, if the items array contains
   simple strings (likely typeahead suggestions)
 - to the mention.js render function, in all other cases
   (more complex non-string item objects, likely for mention.js)

Before this, using typeahead on a page that initialized mention.js was not
possible, due to render() being patched to only support rendering
mention.js items.

This could be improved on more later by adding better/smarter detection.
It probably won't be necessary though, with Bootstrap 3.0 coming soon,
which migrates from its own Typeahead plugin to the completely different
Twitter Typeahead.

Here's a test page that was previously broken:

	<!doctype>
	<html>
		<head>
			<link rel="stylesheet" href="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/2.3.2/css/bootstrap.min.css">
			<link rel="stylesheet" href="mention.css">
		</head>
		<body>
			Typeahead: <input type="text" id="js-textbox-typeahead" />	
			<br />
			Mention: <input type="text" id="js-textbox-mention" />

			<script type="text/javascript" src="http://cdnjs.cloudflare.com/ajax/libs/jquery/2.0.3/jquery.min.js"></script>
			<script type="text/javascript" src="bootstrap-typeahead.js"></script>
			<script type="text/javascript" src="mention.js"></script>
			<script type="text/javascript">
				$('#js-textbox-typeahead').typeahead({
					"source": ["something"]
				});
			
				$('#js-textbox-mention').mention({
					"users": [{"username": "sarah", "name": "Sarah Doe"}]
				});
			</script>
		</body>
	</html>