Remove peer dependency on datasource-juggler

Use `Connector` from `loopback-connector` as the base class for the
MongoDB connector.

Note: the behaviour remains backwards compatible, the connector can be
used with both old 1.x and upcoming 2.x versions of
loopback-datasource-juggler
Merge pull request #34 from strongloop/feature/refactor-connector-base

Remove peer dependency on datasource-juggler