Create Configuration class to store global driver settings

[aleksslitvinovs]

In this PR 3 Configuration class has been added with the following options that can be adjusted via

• NewDriver() setter methods (set_screenshot_path(), set_save_screenshot_on_fail(), set_save_full_stacktrace())
• driver:TestUIDriver instance properties (driver.configuration.screenshot_path, driver.configuration.save_screenshot_on_fail, driver.configuration.save_full_stacktrace).

Additionally, .gitignorehas been updated as using gitignore.io.

[alvarolaserna]

Also, could you add in the Readme file something about the configuration properties?