Add help output for the rb-site manage command.

Review Request #6367 — Created Sept. 23, 2014 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
79ea932...

Reviewers

This includes some basic help output for rb-site's 'manage' command that
explains how to get the list of commands and execute them.

Ran rb-site --help and saw the help content.

Note that we have no control over newlines, which is why the help text isn't
formatted any nicer.