feat(controller): add Domain model

This is added in the API with 2 new URLS:

GET /apps/{app}/domains
    create, get, or edit the domains for an app
POST /apps/{app}/domains/{domain}
    delete a domain.

The CLI is adapted to make use of this new API.

TESTING: rebuild both the router and the controller:

    $ make -C controller build restart
    $ make -C router build restart

install the new client:

    $ cd client && pip uninstall deis && python setup.py install

create an app, push it to deis, and add a domain to the app (note
that you can still utilize the wildcard DNS from deisapp.com so
you don't have to set up DNS records for custom domains):

    $ cd example-ruby-sinatra
    $ deis create
    $ git push deis master
    $ deis domains:add funky-monkey.deisapp.com
    $ curl funky-monkey.deisapp.com
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