Add ability to set per-world entity spawn limits in spigot.yml

This implements a similar function to the spawn-limits sub catagory in bukkit.yml, but it is formed in the spigot.yml's per-world config section, allowing you to set the natural limits (mobs spawned due to light level <7) on a per-world basis.

The bukkit.yml documentation is here http://wiki.bukkit.org/Bukkit.yml#spawn-limits but they work as an overall for the whole server, while this implementation can work on a per-world basis.

The logic used simply replaces the check that if a plugin hasn't set a limit, for example, monsterSpawn (defaults to -1 as set at the top of the source file of CraftWorld.java), instead of checking for the bukkit.yml's overarching server settings, it will grab the specific world's spigot.yml limit.

Screwed up the branch in PR #109, gets defaults from bukkit.yml to allow for conversions.