perf tools: Add example call-graph script

Add a script to produce a call-graph from data exported to a postgresql
database and derived from a processor trace event like intel_pt or intel_bts.

Refer to comments in the scripts call-graph-from-postgresql.py and
export-to-postgresql.py for more details on how to set up the environment,
install the required packages, etc.

Committer note:

From the scripts, for convenience while reading 'git log':

  An example of using this script with Intel PT:

  $ perf record -e intel_pt//u ls
  $ perf script -s ~/libexec/perf-core/scripts/python/export-to-postgresql.py pt_example branches calls
  2015-05-29 12:49:23.464364 Creating database...
  2015-05-29 12:49:26.281717 Writing to intermediate files...
  2015-05-29 12:49:27.190383 Copying to database...
  2015-05-29 12:49:28.140451 Removing intermediate files...
  2015-05-29 12:49:28.147451 Adding primary keys
  2015-05-29 12:49:28.655683 Adding foreign keys
  2015-05-29 12:49:29.365350 Done
  $ python tools/perf/scripts/python/call-graph-from-postgresql.py pt_example
  # The result is a GUI window with a tree representing a context-sensitive
  # call-graph.  Expanding a couple of levels of the tree and adjusting column
  # widths to suit will display something like:

                                         Call Graph: pt_example
  Call Path                        |Object     |Count|Time(ns)|Time(%)|Branch Count|Branch Count(%)
  v- ls
     v- 2638:2638
         v- _start                  ld-2.19.so    1   10074071  100.0        211135          100.0
           |- unknown               unknown       1      13198    0.1             1            0.0
           >- _dl_start             ld-2.19.so    1    1400980   13.9         19637            9.3
           >- _d_linit_internal     ld-2.19.so    1     448152    4.4         11094            5.3
           v-__libc_start_main@plt  ls            1    8211741   81.5        180397           85.4
              >- _dl_fixup          ld-2.19.so    1       7607    0.1           108            0.1
              >- __cxa_atexit       libc-2.19.so  1      11737    0.1            10            0.0
              >- __libc_csu_init    ls            1      10354    0.1            10            0.0
              |- _setjmp            libc-2.19.so  1          0    0.0             4            0.0
              v- main               ls            1    8182043   99.6        180254           99.9

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1437150840-31811-11-git-send-email-adrian.hunter@intel.com


[ Added 'python-pyside qt-postgresql' to the yum cmdline installing required packages ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>