Refactor cover detection

obsolate hardcode %cover_id%= ‘cover’ in <pkg:item name=%cover_id% href=%coverfile%>
now  %cover_id% retrieve from <meta name=“cover” content=%cover_id%>

cps/epub.py

@@ -72,7 +72,9 @@ def get_epub_info(tmp_file_path, original_file_name, original_file_extension):
     if len(coversection) > 0:
         coverfile = extractCover(zip, coversection[0], coverpath, tmp_file_path)
     else:
-        coversection = tree.xpath("/pkg:package/pkg:manifest/pkg:item[@id='cover']/@href", namespaces=ns)
+        meta_cover = tree.xpath("/pkg:package/pkg:metadata/pkg:meta[@name='cover']/@content", namespaces=ns)
+        if len(meta_cover) > 0:
+            coversection = tree.xpath("/pkg:package/pkg:manifest/pkg:item[@id='"+meta_cover[0]+"']/@href", namespaces=ns)
             if len(coversection) > 0:
                 filetype = coversection[0].rsplit('.',1)[-1]
                 if filetype == "xhtml" or filetype == "html": #if cover is (x)html format
@@ -85,12 +87,6 @@ def get_epub_info(tmp_file_path, original_file_name, original_file_extension):
                     coverfile = extractCover(zip, filename, "", tmp_file_path)
                 else:
                     coverfile = extractCover(zip, coversection[0], coverpath, tmp_file_path)
-        else:
-            meta_cover = tree.xpath("/pkg:package/pkg:metadata/pkg:meta[@name='cover']/@content", namespaces=ns)
-            if len(meta_cover) > 0:
-                meta_cover_content = tree.xpath("/pkg:package/pkg:manifest/pkg:item[@id='"+meta_cover[0]+"']/@href", namespaces=ns)
-                if len(meta_cover_content) > 0:
-                    coverfile = extractCover(zip, meta_cover_content[0], coverpath, tmp_file_path)
             
     if epub_metadata['title'] is None:
         title = original_file_name