Bug 1042116 - Add bookmarks provider r=kgrandon

Hi folks,

This PR is for supporting the bookmarks store, took a bit more of time, since I decided to reuse the Places one, since it's basically the same content.

In this PR you can find a generalisation (actually same code) of the Places provider into a new provider called SyncProvider, with Places and Bookmarks using this generalisation.

Thanks!
It seems like ```ready``` might not be the best name for this. Instead can we call this either ```init``` or move this into the constructor and call it from the base class?