Add bearer token support

[elisherer]

This would add bearer token support to the auth method (both for client and for node).

.auth(accessToken, { type: 'bearer' })

Notice I made pass optional since bearer authorization doesn't need a 2nd argument.

[kornelski]

Thanks for the pull request.

@focusaurus what do you think about adding this?

[focusaurus]

I'm OK with it, as long as people don't interpret it to mean some kind of deep OAuth 2 support.