Allow "external" JDocument classes

At the moment all JDocument render classes have to be placed into "libraries/joomla/document". You cannot load your own JDocument child class with e.g. a plugin, because if getInstance does not find the class file in the mentioned path, it fallbacks to JDocumentRaw.

With this patch the getInstance method checks first if the class exists (= e.g. loaded by plugin) and uses the JDocumentRaw class as fallback after the requested class does not exist and could not be loaded.

To test: create an own JDocument child class (like this PDF class: http://docs.joomla.org/J2.5:Creating_PDF_views), load it over a plugin and try to call it with "format=pdf" over the url.
Before the patch you should get the raw output, after you should get the PDF.