Added implementation of verification code.

Based on the implementation in the android app: https://github.com/tasermonkey/android-passwordmaker
Currently produces the same output as the test cases implemented in
https://github.com/tasermonkey/android-passwordmaker/commit/590e8cf18ec156f28a7c75debf9c76b61b0a77f9
but produces different results to the app running on my phone.