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

The Minecraft 1.7 update added two new types of trees that weren't added
in the initial update to 1.7: MEGA_REDWOOD and TALL_BIRCH.

Pulled from PR #979

src/main/java/org/bukkit/TreeType.java

@@ -57,4 +57,12 @@ public enum TreeType {
      * Dark Oak tree.
      */
     DARK_OAK,
+    /**
+     * Mega redwood tree; 4 blocks wide and tall
+     */
+    MEGA_REDWOOD,
+    /**
+     * Tall birch tree
+     */
+    TALL_BIRCH,
 }