Replace connector base with loopback-connector

Remove references to Connector and BaseSQL, connectors should require()
loopback-connector instead of loopback-datasource-juggler.
ModelBuilder: add `prototype.defineValueType`

Add a shortcut for registering a new value type.

The current implementation registers the type in the singleton registry
`ModelBuilder.schemaTypes`.

The API should allow us to to change the implementation to register
the type in the scope of ModelBuilder instance only.
Merge pull request #127 from strongloop/feature/refactor-connector-base

[2.0] Feature/refactor connector base