Correct a small mistake on README.rst
New debuginfo widget to help qtile debug.

  Shows focused window's index, stack or section index ...
  Supports only TreeTab and Stack layouts for now but more will follow.

  Closes #401
Add autosplit parameter to stack layout

  Disabled by default, it makes all stacks
  splitted by default.
Add previous_on_rm param to TreeTab layout

Makes previous window selected on remove instead of first.
Default value: False
Rearrange config options

* stacks argument is now num_stacks to avoid conflict
* several arguments from recent PRs merged to defaults framework

Closes #400
Closes #398
Bind a shutdown key in the default config
new widget: a launchbar

... with icons to click to launch a software or to execute a qtile python
instruction (like for login out, see example in code)

Closes #397