RavenDB-1565 Automatic conflict resolution (local vs remote) 

Note that I had to change the signature of TryResolve method in for AbstractResolvers by adding 'out' parameters. It was required by attachments where I couldn't easily replace the attachment data (byte array, so can't extend or shrink while the size of local/remote attachments can differ). I've changed the signatures for both resolvers: document and attachment to make them consistent.