Add subroutine for normalizing `Authorization` header

Heroku API credentials can be specified in a number of ways:

    Authorization: Bearer <Token>
    Authorization: Basic <base64 encode ":<token>">
    Authorization: Basic <base64 encode "<email>:<token>">

This system adds normalization so at least the first two schemes above,
which are always equivalent, can be grouped so that if a 2FA token is
procured for one, it will stay valid for the other.
Merge pull request #4 from brandur/brandur-normalize-authorization

Add subroutine for normalizing `Authorization` header