Add --group option to enable event grouping.  When enabled, all the
group members information will be shown together with the leader.

  $ perf report --group
  ...
  # group: {ref-cycles,cycles}
  # ========
  #
  # Samples: 7K of event 'anon group { ref-cycles, cycles }'
  # Event count (approx.): 6876107743
  #
  #         Overhead  Command      Shared Object                      Symbol
  # ................  .......  .................  ..........................
  #
      99.84%  99.76%  noploop  noploop            [.] main
       0.07%   0.00%  noploop  ld-2.15.so         [.] strcmp
       0.03%   0.00%  noploop  [kernel.kallsyms]  [k] timerqueue_del
       0.03%   0.03%  noploop  [kernel.kallsyms]  [k] sched_clock_cpu
       0.02%   0.00%  noploop  [kernel.kallsyms]  [k] account_user_time
       0.01%   0.00%  noploop  [kernel.kallsyms]  [k] __alloc_pages_nodemask
       0.00%   0.00%  noploop  [kernel.kallsyms]  [k] native_write_msr_safe
       0.00%   0.11%  noploop  [kernel.kallsyms]  [k] _raw_spin_lock
       0.00%   0.06%  noploop  [kernel.kallsyms]  [k] find_get_page
       0.00%   0.02%  noploop  [kernel.kallsyms]  [k] rcu_check_callbacks
       0.00%   0.02%  noploop  [kernel.kallsyms]  [k] __current_kernel_time
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1358845787-1350-18-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>