Added simple rtotp binary

While I understand that this is a pure library, I find myself often just wanting to generate a token from a salt without too much work. Much like [HTTParty](https://github.com/jnunemaker/httparty/blob/master/bin/httparty) I though a small binary to accomplish this might not hurt.

Do you think an executable like this would be useful? I doubt that the *name* of the executalbe is very useful, but it is almost guaranteed to not cause a conflict :)

Thank you for your feedback.