time: Tick and NewTicker inconsistency

[justinmayhew]

NewTicker will panic if the duration isn't greater than zero, while Tick will return nil.

Ideally Tick would panic as well but that could potentially break some code relying on this inconsistency. This special case should be documented as it's definitely not something you'd expect from reading the current Tick documentation.

[gopherbot]

CL https://golang.org/cl/20031 mentions this issue.