Support "Category" field in metadata, export as <category> in index.xml.

common.py

@@ -278,6 +278,8 @@ def parse_metadata(metafile, **kw):
                 thisinfo['source'] = value
             elif field == 'License':
                 thisinfo['license'] = value
+            elif field == 'Category':
+                thisinfo['category'] = value
             elif field == 'Web Site':
                 thisinfo['web'] = value
             elif field == 'Issue Tracker':

update.py

@@ -314,6 +314,8 @@ for app in apps:
             addElement('icon', app['icon'], doc, apel)
             addElement('description', app['description'], doc, apel)
             addElement('license', app['license'], doc, apel)
+            if 'category' in app:
+                addElement('category', app['category'], doc, apel)
             addElement('web', app['web'], doc, apel)
             addElement('source', app['source'], doc, apel)
             addElement('tracker', app['tracker'], doc, apel)