Added implementation of Euler's totient function

[ipsin]

For anyone working in number theory Euler's totient function can be useful, and I thought it might make a welcome addition to your library. I understand this was unsolicited feature, so no worries if you choose not to incorporate it.

Thanks for your time.

[EMBradley]

The comment on lines 455-456 describes the function incorrectly, although as far as I can tell the actual implementation is correct. The totient function counts all the numbers from 1 to n that are coprime with n, not just the primes.