New plugin loader with class remapping

Remaps guava10/12 for plugin compatibility - no longer need to remap
non-NMS plugins using Guava from Bukkit; Essentials now works unmodified.

A start towards implementing issue #12.

Thanks to:

bergerkiller (Libigot) - https://github.com/AlmuraDev/Libigot/blob/master/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java#L58
md_5 (Interglot) - https://github.com/EcoCityCraft/Interglot/blob/master/src/main/java/org/spigotmc/interglot/App.java