Adds some 1.8 blocks to materials.cfg

(Polished) Granite
(Polished) Diorite
(Polished) Andesite
Slime Block

Issue #197
Adds some icons for 1.8 blocks.

(Polished) Granite
(Polished) Diorite
(Polished) Andesite
Slime Block

Issue #197