Add IEssentialsSpawn API class for Essentials Spawn

This adds an IEssentialsSpawn class and implements it in EssentialsSpawn.

The IEssentialsSpawn class has two methods, getSpawn(String) and setSpawn(String, Location).

Currently, other plugins have no way to get or set spawns for EssentialsSpawn. This commit adds a bare minimum API that allows that.