Make looking for cover more lenient

Looking for cover in epub2 can fail because there are multiple cover entries in meta.

For instance, some PragProg ebooks have the following two lines:

      <meta content="cover-image" name="cover" />
      <meta name="cover" content="cover-image" />

(perhaps to accomodate regex based parsers).

This fix makes the parsing more lenient by just checking that at least one entry has been found, instead of exactly one.