perf tools: Add more documentation to export-to-postgresql.py script

Add some comments to the script and some 'views' to the created database
that better illustrate the database structure and how it can be used.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1443186956-18718-8-git-send-email-adrian.hunter@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>