Replication

Please read this document for background on our approach to replication.

http://docs.strongloop.com/display/DOC/Replication

This includes two new models

 - `Change`
 - `Checkpoint`

[Here is an example app using the api.](https://github.com/strongloop/loopback/blob/feature/replication/example/replication/app.js)

/to @raymondfeng 
/cc @bajtos