Return rows as native Python lists

[MathMagique]

Currently, turbodbc returns rows as a dedicated type created by boost::python. This type is not directly printable and adds confusion. Replace this type with a standard Python list.

In addition, before turbodbc converts result set values to Python objects, there is an intermediate step in converting result set values to a C++ std::vector<turbodbc::optional_field>, which is in turn converted to a Python object. This is pretty costly and unnecessary. Both issues can be fixed in a single effort.