[qtcontacts-sqlite-extensions] Implement generic 2-way-sync adapter

This commit adds a generic 2-way-sync adapter, including the majority
of the logic required to implement 2-way-sync with any arbitrary
sync endpoint (with pure-virtual functions for the fetch and upload
functionality which must be implemented by extenders).
Merge pull request #110 from chriadam/2waysync

[qtcontacts-sqlite-extensions] Implement generic 2-way-sync adapter