Updated to add "wks" and "wk" to parse mapping

The short and default format of weeks is currently wks when outputted. This missing mapping would cause a week outputted to be parsed as 1 second vs (3600 * 24 * 7) seconds when resubmitting the value to be parsed.
Merge pull request #27 from shaundrong/master

Updated to add "wks" and "wk" to parse mapping