Document Entity/PlayerPortalEvents and TravelAgent

Hi, this is the documentation I would have been happy to find when I tried to hook into *PortalEvents.
Feel free to comment, this is my first contribution to the Bukkit project.

* Documented **EntityPortalEvent**'s methods
* Documented **PlayerPortalEvent**'s methods
* Explained what a **TravelAgent** actually is

I was not sure about the {@links} to player, entity, from and to. Tell me if it's too much.