Importing draft implementation of a KABC resource based on Akonadi.

This should allow applications using the traditional KABC API to operate on contact data stored in Akonadi.

Open issues:
- save only implemented synchronously
- error handling missing (marked with TODO)
- config widget fails to select the collection it got from the resource
- config widget shows the Akonadi resource name, not very user friendly
- removal of addressees seems not to work
- distribution lists not implemented yet