Support for Local Verifier/API Endpoint

This is a work in progress, but I wanted to get your thoughts on this. I'm using a YubiHSM and my own internal verification server. I figured that rather than keep a fork of the gem in my vendor/ directory of my app, I'd try to contribute back.

One thing I'm not entirely sure about is how to write a test for that particular situation. I mean, it *should* receive a response back that matches what the YubiCloud gives (see Yubico/yubikey-val), but admittedly I'm not 100% on what i'm doing with regard to test design.

I'm happy to do some more testing on my own, but I imagine that this approach wouldn't break things for the majority of folks using YubiCloud for verification. Good idea? Bad idea?