add indirection for the current time to facilitate testing

[crewjam]

[tstranex]

Thanks for submitting the change. I think it's useful to be able to override the time for testing. However, I think the time function should be a field in the Config rather than a global variable. See tls.Config from the standard package crypto/tls (https://golang.org/pkg/crypto/tls/#Config). I know that currently Authenticate doesn't have a Config parameter but this could perhaps be added in a future breaking change to the API.