Increased accuracy of 1.year by accounting for leap years.  The time extensions are not meant to be super-precise but this seems worth it since otherwise you lose a lot of days doing 40.years.ago.  Closes #1488 [tuxie@dekadance.se]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@1499 5ecf4fe2-1ee6-0310-87b1-e25e094e27de