Zero-size Hotp window

[batrudinych]

For now I can't pass 0 as window size because of that check: var window = opt.window || 50;
I assume that comparing with umdefined should be performed in spite of the notp library is kind of low-level one.
See the PR #41.

[guyht]

Got your PR, and while its a bit of an edge case it does make sense so will merge. I dont recommend setting it to 0 as it will require tight sync for your app.