Improve installation instructions
Die silently
Grok config lazily

We don't need to invoke grok_config explicitly in run anymore. Now it
is called by the config routine at its first call.
Call 'return' at the end of every sub-command routine

This is a good practice that for whatever reason PerlCritic hasn't
bugged me about in these anonymous routines.
Don't try to talk to Gerrit in 'version' if not configured
Refactor warning and error messages

All messages are prefixed by 'git-gerrit[TYPE]: ', where TYPE can be
either INFO, ERROR, or SYNTAX.
Reformat the 'missing configuration' error message
Implement the global --help option

This option shows git-gerrit's manpage and quits.
Fix documentation
v0.011

  [Changes]
  - Improve instalation instructions.

  - Consistently format all warning and error messages.

  - Reformat the 'missing configuration' error message.