Expose a world's travel agent. Fixes BUKKIT-3541.

Currently there is no way to create a travel agent without having your own implementation. This used not to be an issue, as PlayerPortalEvent would always return a valid travel agent. However, now it will return null for custom worlds, and so the default PortalTravelAgent/CraftTravelAgent implementation cannot be used for these worlds. By exposing a world's travel agent, plugins can easily replicate default functionality for custom worlds.

Leaky: https://bukkit.atlassian.net/browse/BUKKIT-3541
CraftBukkit PR: https://github.com/Bukkit/CraftBukkit/pull/1014