Explicitly define copy/move constructors for accessor

`accessor` currently relies on an implicit default copy constructor, but that is deprecated in C++11 when a copy assignment operator is present and can, in some cases, raise deprecation warnings (see #888).  This commit explicitly specifies the default copy constructor and also adds a default move constructor.

include/pybind11/pytypes.h

@@ -388,6 +388,8 @@ class accessor : public object_api<accessor<Policy>> {
 
 public:
     accessor(handle obj, key_type key) : obj(obj), key(std::move(key)) { }
+    accessor(const accessor &a) = default;
+    accessor(accessor &&a) = default;
 
     // accessor overload required to override default assignment operator (templates are not allowed
     // to replace default compiler-generated assignments).