`probe` sleeps `sleep-time` before the first try

[dchelimsky]

Ideally, probe would always try the first time right away, and then sleep before subsequent tries, and right now there is no way to make that happen (besides setting :sleep-time to 0, but then it will try :times-to-try times with no sleep at all).

One solution would be to support a :sleep-before-first-try in params. The question is whether it should default to true or false. Hint: one of those is non-breaking, but the other is what I'd have expected/wanted in the first place.

[dchelimsky]

Fixed in #78