PyO3 0.2.0

Changes

• Added inheritance support #15

• Added weakref support #56

• Allow to add gc support without implementing PyGCProtocol #57

• Refactor PyErr implementation. Drop py parameter from constructor.

• Added subclass support #64

• Added self.__dict__ supoort #68

• Added pyo3::prelude module #70

• Better Iterator support for PyTuple, PyList, PyDict #75

• Introduce IntoPyDictPointer similar to IntoPyTuple #69