Local Verifier Cleanup

I fixed some things with the Local API Endpoint stuff...

* Added instructions to `README.md` on how to use a local verifier. Updated your copyright date, too :)
* The upstream `yubikey` gem uses `:url` as the API endpoint URL, not `:api_url` in the `Yubikey::OTP:Verify` function
* I upped the version to 0.5.0 since this introduces functionality that is backwards compatible. 

If it's cool with you, could you cut a rubygems.org release?