Hotp verify

[batrudinych]

As the Readme says, The allowable margin for the counter. The function will check window codes in the future against the provided token. i.e. if window = 100 and counter = 5 all tokens between 5 and 105 will be checked against the supplied token Default - 50 for HOTP code verification.
As I can see, the library now implies another logic (issue #21 says the same).
I have made a PR#39. Please, take a look if you'll have a chance.

[guyht]

I think it would be better to update the readme than change the functionality of the library