Allow for multiple githooks.group specifications

The githooks.group configuration was being treated as a scalar, so that
only the last instance was being considered. Now it's treated as a list
and all groups defined by all of its instances are taken account of.
Take off TODO.pod from the MANIFEST

This document doesn't contribute much to the main documentation shown in
the https://metacpan.org/release/Git-Hooks and distracts the attention
from the really importante documents.
Change site URL in README

I'm changing http://search.cpan.org by http://metacpan.org.
Improve the tutorial.

This rewrites the user tutorial and adds a server tutorial and a Gerrit
tutorial. We're missing still a tutorial for plugin developers.