Add new API to get a Document Class without requiring a Document Object

Add a new XWiki.getClass(String documentName) method. The idea is that this method doesn't require any rights for accessing the passed Document (as opposed to the

{@link com.xpn.xwiki.api.Document#getxWikiClass()}

method which does require to get a Document object first. This is thus useful in cases where the calling code doesn't have the access right to the specified Document. It is also safe because there are no sensitive data stored in a Class definition.