1147463: Log py.warnings to shutup gobject warning

On startup, we were always getting a gobject warning like:
  /usr/lib64/python2.7/site-packages/gobject/__init__.py:115: Warning:
  Attempt to add property subscription_manager+gui+widgets+CellRendererDate::date
  after class was initialised
    type_register(cls, namespace.get('__gtype_name__'))

This warning is spurious and has been removed in newer versions
of glib. See https://bugzilla.gnome.org/show_bug.cgi?id=698614

But warnings to stdout/stdout are not useful for a gui app
typically launched from desktop app launchers not a shell.
So use the "logging.captureWarnings" method to send those
warnings to the "py.warnings" log.

Add a PyWarningsLoggingFilter class to add a note about the
source of the log messages to the log msg, without changing the
current logging format.
try/expect around logging.captureWarnings

It is only available on python 2.7+, and if
not available the 'py.warnings' logger will
be unused.
Merge pull request #1078 from candlepin/alikins/1147463_log_pywarnings

1147463: Log py.warnings to shutup gobject warning