Improve parsing of covers from epubs

[OzzieIsaacs]

Check parse of epubs with property for Cover

 <metadata>
    <meta name="cover" content="cover-image" />
  </metadata>
  <manifest>
    <item id="id_Images_jpg" href="Images/1234.jpg" media-type="image/jpeg" properties="cover-image"/>
  </manifest>

[OzzieIsaacs]

Check epubs with non jpg files
Check reference blocks like this in epubs:

    <reference href="tyle_9780143038511_oeb_toc_r1.html" title="Table of Contents" type="toc"/>
    <reference href="tyle_9780143038511_msr_cvi_r1.jpg" title="Cover" type="other.ms-coverimage-standard"/>
    <reference href="tyle_9780143038511_msr_cvt_r1.jpg" title="ThumbImageStandard" type="other.ms-thumbimage-standard"/>
    <reference href="tyle_9780143038511_msr_ppl_r1.jpg" title="PPCThumbnailImage" type="other.ms-thumbimage"/>
    <reference href="tyle_9780143038511_oeb_cop_r1.html" title="Copyright" type="copyright"/>