Handle AppEngine time strings that don't have seconds

The previous, broken, parsing assumed that AppEngine time strings always
contain "X second(s)". Turns out that part is optional when it is zero.

Auditors: csilvers

Test Plan:
I made sure that memcache.statistics.oldest_item_age looked reasonable
when running:

echo PASSWORD | ./gae_dashboard_scrape.py -e khanbackups@gmail.com -A s~khan-academy