Remove Prefab system in favor of classes

Due to the nature of Forge injecting blocks/items from the server into the
client, we can allow addon developers to extend our series of custom
classes and registration/resource management is handled for them.

Having addons be able to extend the minecraft classes allows complete
control over the block/item and also allows us to not reinvent the wheel
as was occuring with the prefab system. When a player leaves the server
Forge reverts to a frozen state for the default minecraft materials which
means they handle the cleanup.

Example of custom blocks in use: http://i.imgur.com/up8U4rn.png

The above screenshot proves that we can have multiple instances of
CustomBlock/Item but setting an attribute on one doesn't effect the other
(The glowstone wall has a light output level of 1.0 while the glass has no light
output).

Signed-off-by: Steven Downer <grinch@outlook.com>