Add extended mode for gets

When the `extended` argument is supplied, `get` will not
only return the value, but also the flags and CAS value.

    value, flags, cas = cb.get('key', extended=True)

    cb.get(['foo', 'bar'], extended=True)
    # {'foo': (val1, flags1, cas1), 'bar': (val2, flags2, cas2)}