[2.0] Feature/refactor connector base

This PR superseded #118 and targets the 2.0 branch.

Remove `Connector` and `BaseSQL`, they were moved to `loopback-connector`.

Add `ModelBuilder.prototype.defineValueType` that should be used by connectors like loopback-connector-mysql to register new schema types.

Requires strongloop/loopback-connector#2.

/to @raymondfeng @ritch please review

strongloop/loopback#275