Add support for "issuer" parameter and fix tests

We're looking to use this library at Hootsuite, and need the `issuer` parameter, which displays above the code in Google Authenticator. (For an example, compare the Authenticator entry for codes generated by this library with Github's code. It shows "Github" at the top, and "github.com/kbanman" below.

Additionally, I fixed the unit tests and cleaned up whitespace.