Replaced broken gm_timestamp with utc_timestamp

Correct me if I'm wrong, because I can't believe such issue could be overlooked. Maybe I'm just missing something.

So it seems that `gm_timestamp()` has been lying to us all this time. It claims that it returns GMT timestamp but in fact uses [time.time()](http://docs.python.org/2/library/time.html#time.time) function which documentation doesn't even mention no timezones. In practice (at least in CPython) this function returns timestamps in local system time, which I suppose is a happy coincidence for those having servers in GMT timezone.

Also GMT is generally deprecated in favor of UTC.

This patch fixes both issues.

Note that this change is potentially disastrous if library users were to apply it for existing installations as new timestamps may be lower than the previous ones. There should be at least some kind of warning in the change log or the update notes.