Switch to PyGObject by default (#271)

* cd/offset: remove unused call to gobject.thread_init()

There are no threads involved.

* task: switch to PyGObject

Python-gobject-2 is deprecated.

* travis: update dependencies for PyGObject and GLib-2.0

* README: Add GObject Introspection

whipper/command/offset.py

@@ -23,15 +23,12 @@ import os
 import sys
 import tempfile
 import logging
-import gobject
 from whipper.command.basecommand import BaseCommand
 from whipper.common import accurip, common, config, drive
 from whipper.common import task as ctask
 from whipper.program import arc, cdrdao, cdparanoia, utils
 from whipper.extern.task import task
 
-gobject.threads_init()
-
 logger = logging.getLogger(__name__)
 
 # see http://www.accuraterip.com/driveoffsets.htm