Description of problem:
dnf can't be used to view information about groups.

Version-Release number of selected component (if applicable):
0.4.1-1.git55e6369.fc21

How reproducible:
Always

Steps to Reproduce:
1.Try to view information about some group with dnf group info "Some group" command

Actual results:
Fails when tries to output packages but shows group description correctly.

Traceback (most recent call last):
  File "/usr/bin/dnf", line 35, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 272, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 133, in _main
    result, resultmsgs = cli.run()
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 1290, in run
    return self.command.doCommand(self.base.basecmd, self.base.extcmds)
  File "/usr/lib/python2.7/site-packages/dnf/cli/commands.py", line 879, in doCommand
    return self.base.returnGroupInfo(extcmds)
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 865, in returnGroupInfo
    self.displayPkgsInGroups(group)
  File "/usr/lib/python2.7/site-packages/dnf/cli/output.py", line 1125, in displayPkgsInGroups
    columns=columns)
  File "/usr/lib/python2.7/site-packages/dnf/cli/output.py", line 1071, in _displayPkgsFromNames
    for item in sorted(pkg_names):
NotImplementedError: Unsuported operator

Expected results:
Show same result as yum does.