feat(affix): provide wgAffix

Fine tuned the provider so that it just works out of the box without requiring any external css markup.

I was unable to properly test it, looks like karma has some bug regarding `window.scrollTo`, see https://github.com/karma-runner/karma/issues/345

Added the failing tests anyway, tried a workaround (manually emitting the event, but it did not work either), maybe someone will have a better idea.