Fix/strapperjs

Under Joomla 2.5, jQuery and jQuery UI would be loaded multiple times if you call `jQuery()` and/or `jQueryUI()` multiple times. This PR checks it and makes sure it's only included once.

With the last commit I've chosen to set `$_includedJQuery` and `$_includedJQueryUI` only to true when it really has been loaded. This may come in handy when an external script want to know if it has been loaded by Strapper.