Relax setup.py manual runtime depedendency enforcement

Now, only a warning will be issued if runtime dependencies are unmet.
The previous approach introduced unnecessary complexity and one
additional setup dependency (argparse).
Install dbus-python into virtualenv for Travis

We need to perform a manual "./configure; make" installation for travis
because:

- As dbus-python does not have a setup.py it cannot be installed via
  pip/easy_install:

    http://stackoverflow.com/questions/13365697/install-python-dbus-in-virtualenv

- The system packages should not be used for Travis, see:

    CI Environment uses separate virtualenv instances for each Python
    version. System Python is not used and should not be relied on. If
    you need to install Python packages, do it via pip and not apt.

    http://about.travis-ci.org/docs/user/languages/python/#Travis-CI-Uses-Isolated-virtualenvs
Build gobject via autotools

PyGObject does not build properly via PyPI at the moment.