Return the valid max value for piston direction. Fixes BUKKIT-2191

By returning the following value (7) we remove the need to special
case pistons in any way (other than the original purpose of this
check, which is to ensure pistons have valid data)
Reload ban files when reloading the server. Adds BUKKIT-3470
Compensate for allow-nether/allow-end as false; Fixes BUKKIT-3466

When either of those settings are false, the worlds are not loaded and
therefore will not be targeted for portal exits.  Existing worlds are
iterated directly to avoid defaulting to the first world if a direct
dimension match is not found.

Plugins must also specify exit from custom Bukkit worlds to comply with
original commit: https://github.com/Bukkit/CraftBukkit/commit/2dc2af0

This commit introduces a constant to clarify the dependency on the
CraftBukkit implementation of custom worlds having a dimension offset.
Call BlockPlaceEvent for skulls the same as other blocks. BUKKIT-3406

When the skull BlockPlaceEvent was added it was made so the event
would be called after all the data has been set, however this is a
behavior change that is inconsistent with other BlockPlaceEvents.
Instead, if people wish to get the block data they should schedule
a task.

Relates to: BUKKIT-3438