Added option for simple_detection_mode for plugins/xss.py . Edited _chec...

Added option for simple_detection_mode for plugins/xss.py . Edited _check_xss_in_parameter method to check newly created simple_detection_mode variable. If simple_detection_mode is set to true, w3af will only scan for trivial XSS vulnerabilities. Also added simple_detection_mode variable to the get and set options methods. Tested briefly using manual testing. Plan to work with Andres to write some automated tests
please make this a combo option, so in the future we can have more than one detection mode.