Add UTC timezone designator for datetime regex

Last fix (0.5.7) breaks datetime's like "2014-10-09T10:00:00Z".
More info in ISO8601: http://en.wikipedia.org/wiki/ISO_8601#Time_zone_designators

Only first (8bad0cd) commit is necessary, second one (1b67651) just speedup and simplify regex'es, and third one is for PEP8 (we all love PEP8 =).