Function objects with multiple () operators

[tomaka]

With C++14's generic lambdas, I should provide a way to make this work:

lua.writeFunction("print", [](auto val) { std::cout << val << std:endl });

lua.executeCode("print(true)");    // calls the lambda overload for "bool"
lua.executeCode("print(18)");    // calls the lambda overload for "int"

This would also eliminate the writeFunction with template parameter.