fix #2 dfg-viewer: escape image URIs for OpenLayers

Loading of images with spaces will fail because the URI is not encoded.

Example:
http://www.lwl.org/325dig-download/Lesesaal-Digitalisate/C_Adelsarchive/Ber/Ber.Hss/Ber_Hss_RT_3-5/METS.xml

<mets:fileGrp ID="mets-fgrp-00001" USE="DEFAULT">
<mets:file ID="FID_0000000001_DEF" MIMETYPE="image/jpeg" SIZE="1313391">
<mets:FLocat LOCTYPE="URL" xlink:href="http://www.lwl.org/325dig-download/Lesesaal-Digitalisate/C_Adelsarchive/Ber/Ber.Hss/Ber_Hss_RT_3-5//Ber. Rt. 3-5, 00001.jpg"/>
</mets:file>