Bdd read contenttypegroup

To do::
- [x] Add feature file
- [x] Add sentences definitions to interface
- [ ] merge protected to public change (for subcontexts) on ezsystems/ezpublish-community#131
- [ ] merge enable REST testing ezsystems/ezpublish-community#137
- [ ] merge Rest implementation ezsystems/ezpublish-kernel#827
- [ ] merge helpers ezsystems/ezpublish-community#136
- [ ] merge ContentTypeGroup given steps ezsystems/ezpublish-community#135