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 of intermediaries being able to tell
that two authentication schemes are equivalent and consolidate them. Scheme 2
also has the nice 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.~~ bgentry/heroku-go#13
@kr Ah, +1! I suspected that what I was doing was poor practice.

> (You don't have to wait for the change to be merged upstream.)

But my commits won't even be in that repo yet I think? Do you mean that I should run a `godep update` pointing to my own fork?