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.
Merge pull request #287 from strongloop/feature/connector-registry

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