Fixes for new website

The website of the MGG has changed and thus I needed to include some optimizations so that the app works again.

Additionally major improvements have been made to the internals of the app. It can be easier maintained, since the code was cleaned up, a proper datamodel has been created and parser classes handle the parsing of the website. With the last mentioned improvement, the code can be easily changed, so that it handles another school's timetable.