[nemo-qml-plugin-dbus] Add typedCallWithReturn function to DBusInterface.

Support calling DBus interface methods which return a value. The method
call is asynchronous, once the method completes the return value will
be converted into a JavaScript object and assigned to the specified
property on the specified object.
Merge pull request #4 from amccarthy/fixes

[nemo-qml-plugin-dbus] Add typedCallWithReturn function to DBusInterface