[WIP] Traverse immediate child associations by default

[carolynvs]

• [x] svc-cat describe instance <name> prints child bindings by default
• [ ] svc-cat describe plan <name> shouldn't hide the instances behind --traverse and instead should print the parent class and broker.
• [ ] svc-cat describe class <name> shouldn't hide the plans behind --traverse and should print the parent broker.

Closes #22.