Authorization implementation

[ZoopOTheGoop]

The code is set up to handle authorization easily, but it's not implemented. The general outline is this:

Create a new struct like so:

pub struct AuthorizedClient<'a> {
    underlying: SnapdClient,
    macaroon: Macaroon<'a>
}

Where Macaroon is a newtype over a Cow. Then you can implement GetClient (etc) as for SnapdClient, but override attach_headers to also attach the auth header.

As for getting the credentials, use the /v2/login API, which can be implemented under the hood as a Get, and put a convenience method on SnapdClient called login<'a>(&self) -> AuthorizedClient<'a> {} which creates the client with an owned macaroon.