Add Native Behavior (cpp)

[adderly]

The idea is to create a simple class for easy subclassing.
I have some code to test, it extends from behavior and instead of calling the script it calls a std::function which the user should provide.

Sometimes, doing all the logic from javascript add some extra/unnecessary overhead not only on processing but in memory.

Await for the pull.

@kenvandine i guess providing a clean interface will suffice.