Add fetching of encodedPasskey

[dnschneid]

From dnschneid/pysmartblinds#4, it would be useful to be able to retrieve the encodedPasskey field.

In particular, being able to read out the base64-decoded versions of encodedMacAddress and encodedPasskey would be very useful, but users of the library could do that themselves as well.

[ianlevesque]

Fixed by 424f1b8