[evol] allow passing any parameter to the ruby-mail delivery settings
[evol] implement stopatfirsterror debug flag (was unconditionnal but broken until 7e264b1)
[evol] removed useless 'debug_' prefix in debug flags
[evol] quick code to handle running an external script to reload the zone (needed for signed zones)
[evol] configuration defaults and schema checking (closes 21)