ensure we .check() Checked messages

This is from 71a8d0d04602c53dd05e15a3cf8dde0a00e1fc97, but was inadvertently
removed in a758f11036c4a1f0ad83faf53933098117e6f567; we should keep it.
we use pybuild, so we should depend on dh-python

viz. https://wiki.debian.org/Python/Pybuild
Don't die horribly if xephyr fails to start

As seen in https://travis-ci.org/qtile/qtile/jobs/39096012, if xephyr fails to
start, we generate a long chain of exceptions, because we're using testwindows
before it is initalized. We should initialize it first.
Use to_atoms instead of unpacking struct
Change .get_property(unpack=...) to be int, use to_atoms/string

tych0: minor correction for the bug we were seeing in
1739627aa6534b0a88a45cd98587a52753e857e8.

Closes #536
widget: graph: NetGraph: fix unhandled exception

If Qtile trys to come up without default-route, this TraceBack apears:

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/qtile-0.8.0-py2.7.egg/libqtile/confreader.py", line 54, in __init__
    config = __import__(os.path.basename(fname)[:-3])
  File "/home/fsc/.config/qtile/config.py", line 139, in <module>
    widget.NetGraph(**widget_graph_defaults),
  File "/usr/local/lib/python2.7/dist-packages/qtile-0.8.0-py2.7.egg/libqtile/widget/graph.py", line 285, in __init__
    self.interface = self.get_main_iface()
  File "/usr/local/lib/python2.7/dist-packages/qtile-0.8.0-py2.7.egg/libqtile/widget/graph.py", line 321, in get_main_iface
    routes[0]
IndexError: list index out of range

Signed-off-by: Florian Scherf <fscherf@gmx.net>