Gaia/Contacts/Data Refactor/mozcontactsProposal

mozContacts Datastore as the GlobalContactsDatastore

- The idea is that instead of having an specific GlobalContactsDatastore we use the mozContacts datastore for that purpose. An entry in the mozContacts datastore could have only local data or could also have pointers to provider contacts present in the external providers datastores (Facebook, Gmail, etc.).

The approach described here does not change so much things, it only removes a Datastore, the local Contacts datastore, role, that we've been given till now to the mozContacts datastore. In some sense, we are collapsing two datastores (mozContacts and GCDS) into one.

We will be providing a library that will allow to resolve a mozContact to the real data it should have, as we have been already done with the MultiContact library.