Fix access to embedded status containers

[rytilahti]

Also, implement __dir__ to enable autocompletion for attributes of embedded containers.

Example:

In [1]: from miio import RoborockVacuum
   ...: vac = RoborockVacuum("x", "x")
   ...: stat = vac.status()

In [2]: stat.DNDStatus
Out[2]: <DNDStatus enabled=True end=08:00:00 start=22:00:00>

In [3]: stat.DNDStatus<tab>
                       DNDStatus          DNDStatus__end    
                       DNDStatus__enabled DNDStatus__start