[Pulp-list] Refactored RepoApi.sync() to repo_sync.sync()

[John Matthews]

I've pushed changes to repository synchronization.

1) sync() under RepoAPI has been moved to repo_sync.py
2) "Synchronizer" classes under repo_sync.py have been moved to synchronizers.py
3) repo_sync.py contains the high level logic for sync operations, it will coordinate between RepoAPI and synchronizers.py

unit tests, syncs and clones are looking good, yet I still may have missed something.  If you notice any issues please let me know.