U1db implementation

Even I couldn't find any `sync-to` in the u1db trunk. I don't know what I was thinking then. I even defended this. :P 

I will change it to `sync-from` as your explanation makes sense to me. But I dont really under stand the SyncResource class very well yet. If we go by the zeromq semantics, honestly I haven't found any use for `SyncType` messages in our use case. I didn't understand it initially so was just trying to maintain the existing message structure. I don't have any problems in removing it totally. Do you think it will come in handy later on? Your suggestions on this?