[nemo-qml-plugin-contacts] Don't delete contacts held by the cache

Results in a double-delete abort.
[nemo-qml-plugin-contacts] Listen for contact changes in standalone Person

In a Person object which is not owned by a model, listen for changes to
the cached contact, in case it is deleted or updated.
Merge pull request #30 from matthewvogt/fix-delete

[nemo-qml-plugin-contacts] Don't delete contacts held by the cache