Implemented sortIds ( interface IMetaModelAttribute )
Merge pull request #7 from paulpflug/sortids

Implemented sortIds ( interface IMetaModelAttribute )
Merge branch 'dev' into contao3
Merge branch 'contao3' into tng
Activate ccabs.
Change spaces to tab.
Fix code style issues.
Change directory structure.
Add unit test.
Drop legacy registration and use a factory now.
Introduce new getters and utilize inherited getters for attribute settings.
Utilize convertOptionsList() from base class.
Early exit if not both, source table and source column, are available.
Extract determination of the order by part of getFilterOptions() into own method.
implement getFilterOptionsForUsedOnly($usedOnly).
Rework getFilterOptions() to lazy retrieve values.
Add php 5.6 to .travis.yml