Time limited signature

Duplicates https://github.com/norcross/yourls-link-creator/pull/5 for the benefit of the new repo

This pull request replaces the signature token in the GET request with the "time limited signature token" documented in the tools section of a YOURLS install.  This reduces the damage that could be done if the resulting token and time stamp were somehow obtained by a malicious entity (several examples are mentioned at http://stackoverflow.com/questions/323200/is-an-https-query-string-secure).