Fix IL2CPP.exe crash. Issue #101.

IL2CPP crash when it faces following code:
1) It uses generic contraint with another generic parameter.
2) Using 1, invoking some member via contrained type.
3) The type of generic argument for parameter in 1 is value type instead of reference type.
So, this commit changes ReflectionSerializerNilImplication*Parameter from struct to class in Unity build.

Author: yfakariya

src/MsgPack/Serialization/ReflectionSerializers/ReflectionSerializerNilImplicationHandlerOnUnpackedParameter.cs

@@ -2,7 +2,7 @@
 //
 // MessagePack for CLI
 //
-// Copyright (C) 2014 FUJIWARA, Yusuke
+// Copyright (C) 2014-2015 FUJIWARA, Yusuke
 //
 //    Licensed under the Apache License, Version 2.0 (the "License");
 //    you may not use this file except in compliance with the License.
@@ -18,11 +18,19 @@
 //
 #endregion -- License Terms --
 
+#if UNITY_STANDALONE || UNITY_WEBPLAYER || UNITY_WII || UNITY_IPHONE || UNITY_ANDROID || UNITY_PS3 || UNITY_XBOX360 || UNITY_FLASH || UNITY_BKACKBERRY || UNITY_WINRT
+#define UNITY
+#endif
+
 using System;
 
 namespace MsgPack.Serialization.ReflectionSerializers
 {
+#if !UNITY
 	internal struct ReflectionSerializerNilImplicationHandlerOnUnpackedParameter : INilImplicationHandlerOnUnpackedParameter<Action<object>>
+#else
+	internal sealed class ReflectionSerializerNilImplicationHandlerOnUnpackedParameter : INilImplicationHandlerOnUnpackedParameter<Action<object>>
+#endif
 	{
 		private readonly Type _itemType;
 		public Type ItemType { get { return this._itemType; } }

src/MsgPack/Serialization/ReflectionSerializers/ReflectionSerializerNilImplicationHandlerParameter.cs

@@ -2,7 +2,7 @@
 //
 // MessagePack for CLI
 //
-// Copyright (C) 2014 FUJIWARA, Yusuke
+// Copyright (C) 2014-2015 FUJIWARA, Yusuke
 //
 //    Licensed under the Apache License, Version 2.0 (the "License");
 //    you may not use this file except in compliance with the License.
@@ -18,11 +18,19 @@
 //
 #endregion -- License Terms --
 
+#if UNITY_STANDALONE || UNITY_WEBPLAYER || UNITY_WII || UNITY_IPHONE || UNITY_ANDROID || UNITY_PS3 || UNITY_XBOX360 || UNITY_FLASH || UNITY_BKACKBERRY || UNITY_WINRT
+#define UNITY
+#endif
+
 using System;
 
 namespace MsgPack.Serialization.ReflectionSerializers
 {
+#if !UNITY
 	internal struct ReflectionSerializerNilImplicationHandlerParameter : INilImplicationHandlerParameter
+#else
+	internal sealed class ReflectionSerializerNilImplicationHandlerParameter : INilImplicationHandlerParameter
+#endif
 	{
 		private readonly Type _itemType;