Allowing partial specialization of the convert struct

This is done by adding a second, defaulted, template parameter that can
be used in conjunction of std::enable_if

An example of usage is added to the tutorial

Author: BenjaminNavarro

docs/Tutorial.md

@@ -198,4 +198,74 @@ Then you could use `Vec3` wherever you could use any other type:
 YAML::Node node = YAML::Load("start: [1, 3, 0]");
 Vec3 v = node["start"].as<Vec3>();
 node["end"] = Vec3(2, -1, 0);
+```
+
+## Partial specialization
+
+If you need to specialize the `convert` struct for a set of types instead of just one you can use partial specialization with the help of `std::enable_if` (SFINAE).
+
+Here is a small example showing how to partially specialize the `convert` struct for all types deriving from a base class:
+
+```cpp
+// Base class
+class A
+{
+public:
+	// virtual load/emit methods
+	virtual void load(const YAML::Node &node) {
+		a = node["a"].as<int>();
+	}
+
+	virtual YAML::Node emit() const	{
+		YAML::Node node;
+		node["a"] = a;
+		return node;
+	}
+  
+	int a;
+};
+
+// Derived class
+class B : public A
+{
+public:
+	// override virtual load/emit methods
+	virtual void load(const YAML::Node &node) override {
+		A::load(node);
+		b = node["b"].as<int>();
+	}
+
+	virtual YAML::Node emit() const override {
+		YAML::Node node = A::emit();
+		node["b"] = b;
+		return node;
+	}
+
+	int b;
+};
+
+// Implementation of convert::{encode,decode} for all classes derived from or being A
+namespace YAML {
+	template<typename T> 
+  struct convert<T, typename std::enable_if<std::is_base_of<A, T>::value>::type> {
+		static Node encode(const T &rhs) {
+			Node node = rhs.emit();
+			return node;
+		}
+
+		static bool decode(const Node &node, T &rhs) {
+			rhs.load(node);
+			return true;
+		}
+	};
+}
+```
+
+Which can then be use like this:
+```cpp
+YAML::Node node = YAML::Load("{a: 1, b: 2}");
+B b = node.as<B>();
+b.a = 12;
+b.b = 42;
+node = b;
 ```

include/yaml-cpp/node/convert.h

@@ -27,7 +27,7 @@
 namespace YAML {
 class Binary;
 struct _Null;
-template <typename T>
+template <typename T, typename Enable>
 struct convert;
 }  // namespace YAML
 

include/yaml-cpp/node/node.h

@@ -141,7 +141,7 @@ YAML_CPP_API bool operator==(const Node& lhs, const Node& rhs);
 
 YAML_CPP_API Node Clone(const Node& node);
 
-template <typename T>
+template <typename T, typename Enable = void>
 struct convert;
 }