Enable IsNull() for older versions of MSVC

IsNull() is only available for C++11. However, older versions of
MSVC know nullptr and std::nullptr_r and can also handle IsNull().
This commit uses the workaround proposed by @wisk in issue #17 to
enable IsNull() for MSVC version identifiers >= 1600.

example/basic_assertions.cpp

@@ -184,7 +184,7 @@ void BasicAssertions()
 			"Expected: less than or equal to 5\nActual: 6\n");
 	}
 
-#if __cplusplus > 199711L
+#if __cplusplus > 199711L || (defined(_MSC_VER) && _MSC_VER >= 1600)
   std::cout << "ShouldHandleNull" << std::endl;
     {
        Assert::That(nullptr, IsNull());

snowhouse/constraints/equalsconstraint.h

@@ -51,7 +51,7 @@ namespace snowhouse {
     return EqualsConstraint<bool>(true);
   }
 
-#if __cplusplus > 199711L
+#if __cplusplus > 199711L || (defined(_MSC_VER) && _MSC_VER >= 1600)
   inline EqualsConstraint<std::nullptr_t> IsNull()
   {
     return EqualsConstraint<std::nullptr_t>(nullptr);