[API add] Add API for checking pygobject's version

Add a gi.__version__ attribute for the textual version, and gi.version_info for
a version triple similar to sys.version_info.

Also add a gi.require_version(<minimum_version>) which raises an exception if
the pygobject version is older.

https://bugzilla.gnome.org/show_bug.cgi?id=680176



Signed-off-by: Martin Pitt <martinpitt@gnome.org>