make it more obvious that title is WM_NAME
group spawn arg, starts the client in that group

it uses net_wm_pid so the client should implement it, but most does

Closes #442
Fix screen mirroring while respecting ordering

Closes #439
widget/wlan.py:

Fix for IOError when wlan device is deactivated (Softswitch)

Closes #438
Fix a test

apparently int objects aren't marshallable, so we have to return a string here.
Fix #443

Looks like these functions were removed by
d87967ed4001d8f222c24827b43b31240795be4c and not tested so nothing caught it
:-(
clock widget supports other timezones
Add background gradient documentation
stack: 'return' in configure for windows not owned by this layout

The first check in the configure function tries to find the window in
the stacks, and if it's not found, it hides it. This commit adds a
return after hiding it, because there's no reason to continue
configuring it.
drawer: cleanup - turn draw_fill into draw(fill=True)

The body of both functions looked copypasted.
group: Catch and avoid propagating exceptions in layout()

Just a way to prevent misbehaving layouts from breaking more than they
should.
default_config: Stack has a 'num_stacks' param, not 'stacks'
add support to display the volume(widget) states as emoji characters
Don't import ThermalSensor twice
Drop Xlib from tests, use xcb

Closes #447
Move cmd_(eval|function) to all objects of the ipc tree

They were in the manager object only previously, now they are in
CommandObject, which means all subclasses have it (including manager)

Now it's possible to use eval() from any part of qsh, for example:

    group['term']> eval("self.windows")
    (True, 'set([Window(st)])')

Or have keybindings that work on a specific kind of object:

    Key([mod], "x", lazy.window.function(lambda w: w.setOpacity(0.5))),

Figuring out any useful things to do with these is left as an exercise
for the reader.
Clean up unused imports
Enable travis tests
Travis builds don't have SwapCached in meminfo
Add qtile Travis-ci status to README