run database_cleaner before suite

- clean before all the tests are run
- does not change how clean happens between tests
- clean between tests is skipped in some 'sloppy' cases where database cleaner doesn't expect
- added comments and examples of skipped 'sloppy' specs
- maybe more config or replace db cleaner would even get those
- travis deletes db between runs so is not a good example whether db cleaner works between runs