Implement a .pyi stub file to implement optional type hinting (usable with mypy or Python >= 3.5) as initially proposed by @CrazyPython.

See #198 (comment).

TODO:

• Implement type hint .pyi stub files (tutorial here and autogeneration tools here).
• Test statically using MyPy?
• Add MyPy static test to Makefile
• Automate MyPy test in Travis

Note that the official implementation of type hints in Python 3.5 may change in Python 3.6/3.7, particularly for duck typing support, but since it's an optional stub file (no modification to .py files) we won't lose anything.