Remove the '--editor' option from sub-commands

The --editor option does not exist anymore. The Git editor will always
be invoked if the --message isn't present. This is similar to
git-commit's interface. It's still possible to not add any message by
specifying an empty message (e.g., '--message ""') to the
sub-commands. This means that git-gerrit doesn't die anymore if empty
messages are specified.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Fix spelling

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Improve documentation

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Turn git-gerrit into a proper CPAN distribution

I followed the suggestion at
http://elliotlovesperl.com/2009/11/23/how-to-structure-perl-programs/
and turned the git-gerrit script into a module called Git::Gerrit with
a 'run' exported routine. Now, the script just uses the module and
invokes 'run'. This allowed me to turn it into a proper CPAN
distribution, making it easier to install and updated it.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>