[nemo-qml-plugin-contacts] Include bluetooth contacts in export

Contacts with the 'bluetooth' sync target are not actually managed
by an external service, but are effectively local device data once
they have been imported.  They should therefore be backed up and
restored along with other local contacts.
Merge pull request #108 from matthewvogt/bt-backup

[nemo-qml-plugin-contacts] Include bluetooth contacts in export