Feature/add update

This PR adds two new features:

1. like/nlike support for memory connector
2. Add a new API for update(where, data, cb)

/to @ritch 
/cc @bajtos 
A couple concerns if we want to release this in 1.x...

What if they are using a connector that doesn't have this implemented (eg.`loopback-connector-mongodb@1.3.0`)?

If their package.json has `loopback-datasource-juggler: ^1.5.0` a user might see this feature in the docs, run `npm update` to get the right version of the juggler, but never realize they need to upgrade their connector. If the connector is listed as `^1.3.0` then it wouldn't be an issue. Just want to make sure we are thinking about this though.