added basic appium support.

config example (or set these values in your config.yaml):

Watirmark::Configuration.instance.defaults = {
    webdriver:  'appium',
    appium_server:  '0.0.0.0',
    appium_port:  '4723',
    appium_platform:  'iOS',
    appium_version_number:  '8.1',
    appium_device_name:  'iPhone Simulator',
    appium_app_path:  APP_PATH,
}