Change how xlmns, xlink, xsi, and schemaLocation attributes are generated

Changed the way the above attributes, which are used in <modsCollection> or <mods>,
are generated by the xslt. The xlink and xsi namespace are now defined inside the
<stylesheet> tag, and the default namespace and the xsi:schemaLocation attribute are
set using <xsl:attribute> inside the <modsCollection> attribute.