Introduce yourls_deprecated_function()

git-svn-id: http://yourls.googlecode.com/svn/trunk@843 12232710-3e20-11de-b438-597f59cd7555
Oops, s/@since 1.6.1/@since 1.6/. I COMMIT IN THE FUTURE!

git-svn-id: http://yourls.googlecode.com/svn/trunk@844 12232710-3e20-11de-b438-597f59cd7555