Introduce scalastyle

This branch finally reached a level to open it for review.

Lately I fixed some things in Scalastyle itself and its Maven plugin in order that the fulfill our needs. I also added a rule to check the changes done in PR #342. This rule can be activated once this PR is merged.

The Scalastyle introduction can be found in the latest commit. For the beginning only rules that check for trailing whitespaces and tabs are activated. More rules can be activated by time.

The remaining commits either contain whitespace changes or some changes to existing tests that had errors Scalastyle found.