Drop nav and audio elements

<gap> would be much better here, but is not allowed everywhere according
to the I5 DTD :/
diff --git a/xslt/epub2i5.xsl b/xslt/epub2i5.xsl
index 3fc320f..5105147 100644
--- a/xslt/epub2i5.xsl
+++ b/xslt/epub2i5.xsl
@@ -378,10 +378,18 @@
         </p>
     </xsl:template>
 
+    <xsl:template match="xhtml:nav">
+        <!-- <gap reason="toc"/>  -->
+    </xsl:template>
+
     <xsl:template match="xhtml:img">
         <!-- <gap reason="image"/>  -->
     </xsl:template>
 
+    <xsl:template match="xhtml:audio">
+        <!-- <gap reason="audio"/>  -->
+    </xsl:template>
+
     <xsl:template match="xhtml:a">
         <ref target="{@href}">
             <xsl:apply-templates />