string.NiceTime improvements.

Fixed it so past 24 hours it displayed #days, also added weeks and years as this is a nice little function that can be used in ban systems etc.. ( Not sure if months will work as some months have 4 or 5 weeks in them )

Also made it pluralize things correctly, 1 seconds is no more. Example usage: https://eval.in/87215
Fixed indentation.

GitHub web editor has funky indentation, oops.
Merge pull request #552 from HandsomeMatt/fix_nice_time

string.NiceTime improvements.