[time.rb] added method to parse ISO8601 dates that contain timezones

According to http://www.unicode.org/reports/tr35/tr35-25.html#Date_Format_Patterns from http://developer.apple.com/library/ios/#documentation/cocoa/Conceptual/DataFormatting/Articles/dfDateFormatting10_4.html
[time_spec.rb] added spec for parsing ISO8601 Date string's that contain the full timezone. revived spec for timezones.
Merge pull request #255 from bogardon/master

IS8601 Date Formats