Slabs don't work in alvearys

[Daniel Salas]

I think the title is self explanatory. I don't know if this issue comes from Extra Trees or from Forestry (I think only Forestry slabs work on alvearys), but since this is a Forestry addon, I think it's pretty important.

Also you will be able to have red and white alvearys for the merry bee. That will be just hilarious.

[Alex Binnie]

Implemented as of 1.8-pre2