Extend the client configuration plugin architecture.

Plugins can now run code at three key points:
- Before arguments are processed and execution is dispatched to a command.
- After argument processing and dispatch, but before execution.
- After execution.

This allows plugins to perform initialization required for argument processing,
and for post-execution cleanups and synchronizations.

Bugs closed: aurora-332

Reviewed at https://reviews.apache.org/r/20521/
Add a "get version" API method. (This is needed for a logging-related change for twitter.)

While I'm in there, fix a bug in "job list".

Reviewed at https://reviews.apache.org/r/20490/