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

README.md

@@ -78,7 +78,8 @@ Whipper relies on the following packages in order to run correctly and provide a
 - [cd-paranoia](https://www.gnu.org/software/libcdio/), for the actual ripping
   - To avoid bugs it's advised to use `cd-paranoia` **10.2+0.94+2-2**
 - [cdrdao](http://cdrdao.sourceforge.net/), for session, TOC, pre-gap, and ISRC extraction
-- [python-gobject-2](https://packages.debian.org/en/jessie/python-gobject-2), required by `task.py`
+- [GObject Introspection](https://wiki.gnome.org/Projects/GObjectIntrospection), to provide GLib-2.0 methods used by `task.py`
+- [PyGObject](https://pypi.org/project/PyGObject/), required by `task.py`
 - [python-musicbrainzngs](https://github.com/alastair/python-musicbrainzngs), for metadata lookup
 - [python-mutagen](https://pypi.python.org/pypi/mutagen), for tagging support
 - [python-setuptools](https://pypi.python.org/pypi/setuptools), for installation, plugins support