Update CSSNodeFree for C#, Java and Objective-C

Summary:
- Update CSSNodeFree to unlink parent and children for C#, Java and Objective-C bindings finalizer.
- [C#] Fix build (Fix #232)
- [C#] Add Clear API for convenience as CSSNodeFreeRecursive.
- [C#] Revise and add unit tests

Reviewed By: emilsjolander

Differential Revision: D4069655

fbshipit-source-id: 1fd764059784d7968af38b6aaf7fb6f70fdee8ee

csharp/Facebook.CSSLayout/CSSNode.cs

@@ -15,7 +15,7 @@ using System.Text;
 
 namespace Facebook.CSSLayout
 {
-    public class CSSNode : IDisposable, IEnumerable<CSSNode>
+    public class CSSNode : IEnumerable<CSSNode>
     {
         private IntPtr _cssNode;
 
@@ -35,7 +35,6 @@ namespace Facebook.CSSLayout
             {
                 throw new InvalidOperationException("Failed to allocate native memory");
             }
-
         }
 
         ~CSSNode()
@@ -483,6 +482,14 @@ namespace Facebook.CSSLayout
             Native.CSSNodeRemoveChild(_cssNode, child._cssNode);
         }
 
+        public void Clear()
+        {
+            while (_children.Count > 0)
+            {
+                RemoveAt(0);
+            }
+        }
+
         public int IndexOf(CSSNode node)
         {
             return _children.IndexOf(node);