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/cd.py

@@ -24,7 +24,6 @@ import os
 import glob
 import sys
 import logging
-import gobject
 from whipper.command.basecommand import BaseCommand
 from whipper.common import (
     accurip, config, drive, program, task
@@ -32,8 +31,6 @@ from whipper.common import (
 from whipper.program import cdrdao, cdparanoia, utils
 from whipper.result import result
 
-gobject.threads_init()
-
 logger = logging.getLogger(__name__)