feat(client): add apps:rename command

This allows users to modify their app names after having created
one.

There was a previous test in api/tests/test_app that checked
to ensure that the PATCH method was not allowed, so I repurposed
it to test appname changes.

The API call to modify app names is:

    PATCH /api/apps/{appname} {new_name}

Which will return HTTP status 200, and the endpoint is no longer
available. You will be able to reach the app at /api/apps/{new_name}
moving forward.

TESTING: Since this affects practically every aspect of app
management, running the integration test suite is recommended:

    $ vagrant up
    $ make pull
    $ cd test
    $ bundle exec rake

fixes #475