Added SpawnReasons for nether portals and dispensers. Fixes BUKKIT-3148

Previously any entities spawned through dispensers (monster eggs) or
by nether portals were given the incorrect SpawnReason of SPAWNER_EGG.
This made it impossible to distinguish what exactly happened in regards
to the creature being spawned.

With the additional two SpawnReasons, plugins can identify sources of
creature spawning more easily and accuratly.