Add support to get stat events data in perf python scripts.

The python script shall implement the following new interface to process
stat data:

  def stat__<event_name>_[<modifier>](cpu, thread, time, val, ena, run):

    - is called for every stat event for given counter,
      if user monitors 'cycles,instructions:u" following
      callbacks should be defined:

      def stat__cycles(cpu, thread, time, val, ena, run):
      def stat__instructions_u(cpu, thread, time, val, ena, run):

  def stat__interval(time):

    - is called for every interval with its time,
      in non interval mode it's called after last
      stat event with total measured time in ns

The rest of the current interface stays untouched..

Please check example CPI metrics script in following patch
with command line examples in changelogs.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Kan Liang <kan.liang@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1452028152-26762-8-git-send-email-jolsa@kernel.org
[ Rename 'time' parameters to 'tstamp', to fix the build in older distros ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>