Add option to run a window manager

Allow run a window manager for UI tests running using virtual display
feature. If the window manager command specified in the properties file
is found then it will be run before open any browser window. This will
allow the browser window be maximized.
Something like this? What do you think @omaciel?

> Optionally, set the ``window_manager_command`` option in the configuration
> file. This option should be set to the name of a window manager executable.
> For example, ``fluxbox`` or ``openbox`` are suitable values if you have
> Fluxbox or Openbox installed, respectively. Setting this option causes a
> window manager to be launched before the web browser. This allows the web
> browser to be maximized.