Added Big Mode as a piece type. (#20)

Survival A3 and Phantom Mania 2 are now in their fully complete glory! :D

Implements #13.

tetris/rulesets/cambridge.lua

@@ -16,6 +16,16 @@ CRS.spawn_positions = {
 	Z = { x=4, y=4 },
 }
 
+CRS.big_spawn_positions = {
+	I = { x=2, y=2 },
+	J = { x=2, y=3 },
+	L = { x=2, y=3 },
+	O = { x=2, y=3 },
+	S = { x=2, y=2 },
+	T = { x=2, y=3 },
+	Z = { x=2, y=2 },
+}
+
 CRS.block_offsets = {
 	I={
 		{ {x=0, y=0}, {x=-1, y=0}, {x=-2, y=0}, {x=1, y=0} },