Update memcache_report.py to match App Engine UI

Summary:
This required 3 fixes:
1) Some extraction required handling <td>Some <span>text</span></td>
   rather than just <td>Some text</td>.
2) The "oldest item age" format is no longer only "second(s)".
3) Total cache size no longer lists "byte(s)".

Test Plan:
  $ ./gae_dashboard_curl.py 'https://appengine.google.com/memcache?app_id=s~khan-academy' chris@khanacademy.org >memcache.html
  $ <memcache.html ./memcache_report.py -vn 0

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D8024