Added casting constructors to *_wrapper

Added casting constrictors to the *_wrapper structs, so you don't need to use them in user code. This makes the API for the no-boost version compatible with the master branch, so that the example code actually works.

include/mstch/mstch.hpp

@@ -107,18 +107,26 @@ using array  = std::vector<node>;
 
 struct array_wrapper {
     array values;
+	
+	array_wrapper(const array &values) : values(values) {}
 };
 
 struct map_wrapper {
     map values;
+	
+	map_wrapper(const map &values) : values(values) {}
 };
 
 struct object_wrapper {
     std::shared_ptr<object> values;
+	
+	object_wrapper(const std::shared_ptr<object> &values) : values(values) {}
 };
 
 struct lambda_wrapper {
     lambda values;
+	
+	lambda_wrapper(const lambda &values) : values(values) {}
 };
 
 std::string render(const std::string& tmplt,