Move minitests to rspec

This Branch moves most of the Minitests to Rspec. The only ones I didn't touch are Ontology- and Oops-related.

As travis shows, there are only 102 tests (173 assertions) left in Minitest.

In order to make all specs pass, I already cherry-picked 527fefdad77ee24cde5825aaadedf4265da775aa from #1076 on top of this branch.