Add type annotations into library

[eirenik0]

Python type hints allows simplify development of the sdk itself and the visual tests for end users.

What is included:

• python 2&3 compatible type annotations
• tox test runner. It helps to run local tests with different python version
• added flake8 code style test and mypy type check (as mypy needed python 3.6 we set up running flake with mypy on python 3.6 instance)

Here's result of test run https://travis-ci.org/Infernion/eyes.selenium.python/builds/375894756