Periodic bmark sync

Chrome extension implementation should follow this. However, that requires usage of chrome.storage as outlined in https://github.com/bookieio/bookie/pull/345.

https://docs.google.com/file/d/0B1C0uR7ajU7ndFc3Y1hZUGxnejA/edit?pli=1

The above link contains the xpi file for quick testing.

@widox, @mitechie