Strftime

Hi Cauê,

I've add a rough TzDate::formatAs implementation, which isn't perfect, but mostly works. Its does not support `%Z` as I think it will need to rely on the IANA timezone database. 

`%U` and `%W` are not leap year safe and `%W` just does the same as `%U` which can give the wrong result.

I based the codes on http://strftime.org, so any thoughts about improvements are appreciated. I'm happy to see a pure Haxe implementation of utc and tz dates, so thanks for the code :smile:.