Creates an etcd client.

def etcd(url=DEFAULT_URL, mock=False, **kwargs):
    """Creates an etcd client."""
    if mock:
        from etc.adapters.mock import MockAdapter
        adapter_class = MockAdapter
    else:
        from etc.adapters.etcd import EtcdAdapter
        adapter_class = EtcdAdapter
    return Client(adapter_class(url, **kwargs))