Add pyi stubs for better static analysis

[chadrik]

Hi,
I've written pyi stub files for this project that I'd like to submit to the python typeshed, which is a collection of type data about the stdlib and third party packages that is used by static type analyzers like mypy and PyCharm. As per their instructions, I need written sign off from the author in order to contribute these.

As an alternative to adding the stubs to the typeshed, they can be added to the project and released through the normal distutils mechanism: https://www.python.org/dev/peps/pep-0561/

Related to #257

Thanks!

[jmadler]

Thanks for doing this! All else being equal, I would prefer the type hints are colocated with the code itself. Could you submit a patch to include this information in the project?

I'll include it in a release in the next few days.