Add auto_convert parameter to containers

[marcmengel]

The way pybindgen currently works you frequently have to call list(obj.some_method()) when the C++ some_method returns std::vector<> or other container type, to get a fully functional list.

This patch adds an auto_convert parameter to the container that automatically does that for you.
So if you were doing:

mod.add_container('std::vector<std::string>','std::string','vector')

but always doing list(value) for those values, you can instead:

mod.add_container('std::vector<std::string>','std::string','vector',auto_convert="PyList_Type")

and wrapped methods returning that type will always be converted to a list().

Unfortunately this does not happen recursively.