Test DateTimeOffest parsing of partial dates.
Changed DateTime._DoParser to support DateTimeOffset patterns.
Created DateTime.CoreParseExact so to be used by DateTimeOffset.
Improved DateTime parsing of years.
Years with 5 or more digits and less than 1000 are now correctly parsed.
Removed line causing problems when 'K' was followed by something.
Improved DateTime._DoParse offset logic.
Changed DateTimeOffset.ParseExact to use DateTime.CoreParseExact.
Removed unreachable code.
Merge pull request #1234 from esdrubal/datetimeoffset

DateTime.CoreParseExact.