Fixed Mustache AMD definition

This pull request tries to solve compatibility errors that occur when RequireJS (or CommonJS) is loaded on pages where admin/mustache.js is also loaded (for example, in /wp-admin/post.php when a connection box is shown).

The fix is based on how jQuery implements AMD and defines the jQuery and $ global identifiers:

```javascript
// Define a local copy of jQuery
var jQuery = function( selector, context ) {
	...
};

...

if ( typeof module === "object" && module && typeof module.exports === "object" ) {
	// Expose jQuery as module.exports in loaders that implement the Node
	// module pattern (including browserify). Do not create the global, since
	// the user will be storing it themselves locally, and globals are frowned
	// upon in the Node module world.
	module.exports = jQuery;
} else {
	// Register as a named AMD module, since jQuery can be concatenated with other
	// files that may use define, but not via a proper concatenation script that
	// understands anonymous AMD modules. A named AMD is safest and most robust
	// way to register. Lowercase jquery is used because AMD module names are
	// derived from file names, and jQuery is normally delivered in a lowercase
	// file name. Do this after creating the global so that if an AMD module wants
	// to call noConflict to hide this version of jQuery, it will work.
	if ( typeof define === "function" && define.amd ) {
		define( "jquery", [], function () { return jQuery; } );
	}
}

// If there is a window object, that at least has a document property,
// define jQuery and $ identifiers
if ( typeof window === "object" && typeof window.document === "object" ) {
	window.jQuery = window.$ = jQuery;
}
```