Selene

- fixed 71: weird paths of screenshots for windows
- updated 56: now selene should work with python 3 (but feature is not fully tested)

- added selene.tools.wait_to to wait for driver conditions like have.title, have.js_returned_true
- added Title and JsReturnedTrue webdriver conditions
- added selene.tools.execute_script

- refactored conditions implementation
- broken support for python 3 (will be fixed in next versions)

- added automatic screenshots on failed "should" methods
- by default screenshots are created in {user_home}/.selene/screenshots/{id_of_current_tests_run}
- by default the "previous run" screenshots are not cleared on "next run"
- screenshot can be created manually by `selene.tools.take_screenshot`

- improvements:
- error messages are cleaner
- TODO: still lacks proper test coverage of all cases...
- internal
- refactored wait_for implementation, made it cleaner
- TODO: still need to refactor condition implementation

- new features:
- automatic driver management (thanks to PR from SergeyPirogov)
- no more need to `set_driver`,
just use any command from `selene.tools`,
like `visit`, `s`, or `ss` and driver will be opened automatically,
and then closed automatically (unless you decide to set it manually via `set_driver`)
- includes automatic installation of needed drivers via [webdriver_manager](https://github.com/SergeyPirogov/webdriver_manager)