Invoke pip only after building sdist in .travis.yml
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.
Add test for udiskie.common: DBusProperty, DBusProxy

These tests are somewhat ill-defined and could fail on other platforms
if the requested services are not available. In this case they should be
improved to use only defined services.
Add some more documentation in udiskie.udisks
Export only entry-points from udiskie.cli by default

The other methods can still be explicitly exported, but they are not
part of the public API.
Strip module level dependency on gobject, udisks, dbus in udiskie.cli

- allow to inject udisks in all three entry point functions
- gobject is only needed for daemon, so let's inhibit anybody from
  using the others if this library is not available
- dbus is mainly only required to initialize udisks so have it
  instanciated only if udisks needs to be instanciated as well
Install ubuntu-system-service for Travis testing

This package provides the org.freedesktop.hostname1 service that is used
to test udiskie.common.
Install problematic dependencies in before_install

Use the virtualenv --system-site-packages option to inherit these
hard-to-build dependencies into the virtual python environment.

Rationale:
PyGObject and dbus-python turn out to be pretty hard to install using
the virtualenv environment, especially on python3 where the configure
script fails to find the python headers.
Use python-dbusmock for testing

This is better than relying on an external service that may not be
available. Still, the dbusmock library seems somewhat clumsy. It might
be best to write a (very simple) dbus framework.
Merge branch 'testing'

Some basic testing capability for python{2.7,3.3} has been added.