bearer token support

This PR does two simple things:
* look for an 'authToken' field in the sign-in response, and
    store this as hoodie.account.authToken
* send an 'Authorization' header with 'Bearer ' + that token in
    each request to the hoodie-server.

This PR goes together with https://github.com/hoodiehq/hoodie-server/pull/288
on the hoodie-server repo, which at the HAPI level, will:
* strip the couchdb cookie and add it into the JSON response of
    signIn requests.
* read the Authorization header to look for a token of the form
    'Bearer ...', and create a cookie header from the '...', to
    send to the couchdb layer.