Add event registration and option to use proxy.

[leopold-franz]

• Added the option to use a http proxy to download the webdriver and for selenium.
• Added the functionality to register to one-time events (For example Schwimmen - Crawl 3)
• Changed __wait_for_free_places to check number in the Freie Plaetze section instead of checking for "ausgebucht" alert (this option didn't work for events)
• Adapted __get_lesson_time to just get date and time of first lesson listed in Lektionen section.
• Adapted __get_enrollment_time to just use the current date if no "Einschreibe Datum/Zeit" section is found.
• Added 3 tests for webdriver problems.