[boost] shifted_ptr w/ make_shifted_ptr

Ok, I replaced former macro with make_shifted_ptr<>() constructor and allocator function. Like previously mentionned the template parameters of that function add the "const" modifier; therefore it cannot emulate constructors having non-const references. I was reading about the new variadic template arguments that was added in the new compiler... I haven't tried it yet but I am afraid the same problem will happen. Furthermore, I decided not to replace type of "owned<>::p_" with "optional<>" because it doesn't reflect the same purpose and adds an unwanted boolean flag. Regards, -Phil