Timezone support

I added support for timezones in DateTime literals: `datetime'2014-01-02T3:04+01:30'`. 

If the timezone was not provided, the behaviour is unchanged: the DateTimes are returned as is with `DateTimeKind.Unspecified`.

If the timezone was provided (either `Z` for UTC or `+/-HH:MM` for UTC offset), the DateTime is adjusted to the UTC timezone and returned with `DateTimeKind.Utc`.