Add snow to excavation. No treasures will drop from snow by default,

but they can be added to the treasures.yml file by specifying

"Drops_From:
	Snow: true"

Adds #1229

src/main/java/com/gmail/nossr50/util/BlockUtils.java

@@ -180,6 +180,8 @@ public final class BlockUtils {
             case GRAVEL:
             case MYCEL:
             case SAND:
+            case SNOW:
+            case SNOW_BLOCK:
             case SOUL_SAND:
                 return true;