Support std::function

[jjrv]

JavaScript callbacks could perhaps be converted automatically when the C++ parameter type is something like std::function<void(Node*)> lambda.

Plain function pointers probably won't work because we need state to hold a reference to the received JavaScript function.

[jjrv]

This should now work on the Node.js target in the master branch. Still working on asm.js and reflection.

[jjrv]

This is now implemented in the latest release 0.3.9.