app: implement `connector()` and `connectors`

Allow browserified applications to explicitly register connectors
to use in data-sources via `app.connector(name, exportsFromRequire)`.

Include built-in connectors like `Memory` and `Remote` in the registry.

Modify `dataSourcesFromConfig()` to resolve the connector via
`app.connectors` first and only then fall back to auto-require
the connector module.

/to @ritch please review. This change will allow full-stack apps to register datasources via `app.dataSource` without ugly hacks, see your [comment](https://github.com/strongloop/loopback-example-full-stack/pull/7#discussion_r13102382).