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/extern/task/task.py

@@ -21,7 +21,10 @@
 import logging
 import sys
 
-import gobject
+try:
+    from gi.repository import GLib as gobject
+except ImportError:
+    import gobject
 
 logger = logging.getLogger(__name__)