Added support for more meta data

* Added: Support for custom meta data via the function
->addCustomMetadata($name, $content);
*  It is the responsibility of the builder to ensure no invalid values
are inserted.
*  Example use is for for instance Calibre, see EPub.Example250.php for
use.
* Added: Support or user handled DublinCore meta data via the function
->addDublinCoreMetadata(DublinCore::constants, $content);