Patch allows use of XML namespaces

In File_MARCXML allow the use of namespaces. Class File_MARCXML takes two more parameters:

* @param string $ns URI or prefix of the namespace
* @param bool $is_prefix TRUE if $ns is a prefix, FALSE if it's a URI; defaults to FALSE

Example usage

```php
$marc_file = new File_MARCXML('namespace.xml',File_MARC::SOURCE_FILE,"http://www.loc.gov/MARC21/slim");

$marc_file = new File_MARCXML('namespace.xml',File_MARC::SOURCE_FILE,"marc",true);

```