Fix daylight savings time parsing

Use `-secondsFromGMTForDate:` instead of `-secondsFromGMT`.

This was exhibiting different behavior for dates in the past, when the daylight savings time may have been different.

e.g., `2013-01-16 20:00:00Z` would have said: `12:00:00-0700` (which is `19:00:00Z`), when it should have said: `12:00:00-0800`.

Refs #6