add file_mtime_age_ok, file_mtime_lt_ok, file_mtime_gt_ok and associated tests. Update copyright year.

Per Issue #8, here's the pull request. I had some issues with my vim fixing up some incorrect whitespacing in the current File.pm, so the history on the branch needs to be `--squash`'d if you decide to merge. I have no where to test these changes on windows but [perlport](http://perldoc.perl.org/perlport.html#Alphabetical-Listing-of-Perl-Functions) doesn't mention anything on `stat()` that would indicate we should be worried. Just full-disclosure.

Thanks,

\- Mike