855 proper database cleaning

This fixes #855 and #234: The database is cleaned before the test suite and after each test. Also, the Rake task `db:test:purge` is overwritten to simply invoke `db:migrate:clean` and the stubs are removed because we don't need them in this setup any more.


The rake task `db:purge` originally simply recreated the database structure:
```ruby
task :purge => [:environment, :load_config] do
      abcs = ActiveRecord::Base.configurations
      case abcs['test']['adapter']
      when ...
      when /postgresql/
        ActiveRecord::Base.clear_active_connections!
        drop_database(abcs['test'])
        create_database(abcs['test'])
      when ...
      else
        raise "Task not supported by '#{abcs['test']['adapter']}'"
      end
    end
```