mygpo: Convert remote timestamps to local time (bug 1036)

Since applying the first half of this fix (converting the times going
*to* mygpo to utc), I've been losing the sync of some episode actions.
Turns out that it's because now the 'since' value is being stored "in
the future", so the next sync misses some episode actions...

src/gpodder/my.py

@@ -29,6 +29,7 @@ _ = gpodder.gettext
 
 import atexit
 import datetime
+import calendar
 import os
 import sys
 import threading
@@ -417,7 +418,7 @@ class MygPoClient(object):
 
         def convert_from_api(action):
             dt = mygpoutil.iso8601_to_datetime(action.timestamp)
-            since = int(dt.strftime('%s'))
+            since = calendar.timegm(dt.timetuple())
             return ReceivedEpisodeAction(action.podcast, \
                     action.episode, action.device, \
                     action.action, since, \