Add pj() helper function.

The pj() function is helpful in interactive console scenarios, and
general debugging situations as well.

Refs #4791
Merge branch 'pj-function' into 3.0

This is a recreation of pull request #4791. The original pull request
was in a non-mergable state, and I felt it was easier to re-create the
work instead of dealing with tedious merge conflicts.
Merge branch '3.0' of github.com:cakephp/cakephp into 3.0