Add missed tree types for Minecraft 1.7. Adds BUKKIT-5042

This commit adds support for the Tree Types that weren't added in the
initial update to Minecraft 1.7.

Pulled from PR #1277

src/main/java/org/bukkit/craftbukkit/CraftWorld.java

@@ -393,6 +393,12 @@ public class CraftWorld implements World {
         case DARK_OAK:
             gen = new WorldGenForestTree(true);
             break;
+        case MEGA_REDWOOD:
+            gen = new WorldGenMegaTree(true, rand.nextBoolean());
+            break;
+        case TALL_BIRCH:
+            gen = new WorldGenForest(true, true);
+            break;
         case TREE:
         default:
             gen = new WorldGenTrees(true);