Finish Proxy getters for C++ class functions

[guruofquality]

Having function getters let us get a proxy handle to a function pointer for a class. This makes the abstraction more complete and lets the python bindings getattr stuff work more cleanly. These getters should work for a static method or a class method with bound class instance. This is the current work so far however the implementation does not support multiple overloads for the same function. It probably needs to be implemented in the ManagedProxyHandle or overload handling can be done in ManagedClass and the handle can use that -- as long as it not implemented twice.