[nemo-qml-plugin-contacts] Avoid dereferencing null self contact
[nemo-qml-plugin-contacts] Report 'enabled' status of IM account

An IM account is disabled if the telepathy that implements its
transport is not enabled.
Merge pull request #99 from matthewvogt/report-account-enabled

[nemo-qml-plugin-contacts] Report 'enabled' status of IM account