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
Yeah. Add your fork as another remote in `$GOPATH/src/github.com/bgentry/heroku-go`, and make your changes there, so you keep the original import path.

This is not quite ideal, since another person wouldn't be able to run `godep restore`, so it's best when you know there's a good chance the change will be merged upstream. People have also suggested having godep record alternate git sources so that unmerged changes will be reachable by all users, but until then at least it's somewhat ameliorated by having the source code itself present.

(If you want to really fork the project, as in going your separate ways from the original project and never merging upstream, that would be a good time to change the import path.)