Added support etag while parsing entry

Signed-off-by: Sateesh Kavuri <sateesh@mobilitas.co>
Created the structure of the GContactClient plugin class. Added the
necessary abstract method implementations and also the structure for
populating and retrieving the sync results. Next step would be to
implement the startSync method to start the actual sync process

Signed-off-by: Sateesh Kavuri <sateesh@mobilitas.co>
Added GContactsBackend to handle the transactions with the QContact
engine. Need to fix the methods to handle the actual transactions
with GContactEntry

Signed-off-by: Sateesh Kavuri <sateesh@mobilitas.co>