Add nations support to Towny

* config will be updated automatically
* there are now 4 values for `respect_towny`

1. `none` - no Towny checking will take place at all, players can land
in any town
2. `wilderness` - players can only land in wilderness
3. `town` - players can land in wilderness and in any town they are a
resident of
4. `nation` - players can land in wilderness and in any town they are a
resident of, and any town which is in the same nation as the player’s
town