Authenticate like the Heroku CLI

The Heroku API accepts a number of different authentication schemes:

1. `Authorization: Bearer <Token>`
2. `Authorization: Basic <base64 encoded ":<token>">`
3. `Authorization: Basic <base64 encoded "<email>:<token>">`
4. `Authorization: Basic <base64 encoded "<email>:<password>">`

Currently, the CLI uses the scheme 2 while hk uses scheme 3. This pull
changes hk over to the same scheme used by the CLI (that's scheme 2).

Moving over mainly carries the benefit over intermediaries being able to
tell that two authentication schemes are equivalent and consolidate
them. Scheme 2 also has the nice side property of being distiguishable
from scheme 4, which helps intermediaries differentiate token-based
authentication from password-based.

I'll open an equivalent pull on the heroku.go dependency.
Merge pull request #176 from heroku/brandur-authenticate-like-toolbelt

Authenticate like the Heroku CLI