Checking if towny plugin is even there before attempting to use it.
Improved finding of the next SelectableType.
Changed it again how the selectable types are managed.
Attempting to add support for most living entities. Untested yet and
could probably be improved: instead of having to go through all the
available mob types with the creation item, it might be better to just
create a living entity shop of some type the player is allowed to use,
and then given him the option to change the type via the editor menu.

Also a few small things in direction of adding the citizen support (like
some setting) are already included in this commit.

The different entity type settings were removed and replaced with a list
of 'disabled entity types', also the selected messages were replaced
with a generic one.
ShoptTypes, ObjectTypes and uiManagers are no longer automatically
registered on creation, but only on enable (should resolve an issue from
previous commits).
Block some more events of living entities.
Removed some entity types again, as those seem to still move, target and
attack players, even with the current overwriting of entity AI and
blocking of entity target events.

Added chickens to the list of by default disabled living shop types,
because they still spawn eggs (there is currently no good solution to
block this with bukkit). Besides of that however they seem to work fine.