Support JOIN when using relational databases.

[jannyHou]

I create this issue for public discussion of supporting "JOIN" among related models.

[jannyHou]

Related issue:

• Design the query signature: Query on Included Model Properties #1042
• Apply filters on property: Filter by properties of related models (use SQL JOIN in queries) strongloop/loopback#683

[jeff3yan]

Was this previous pull request related? loopbackio/loopback-connector#40