Integrate familiar Alt-Tab functionality in Awesome WM 3.5, tested only on 3.5.5

Features:

* Live previews while alt-tabbing AND/OR Opacity effects for unselected clients
* Easily adjustable settings
* No previews when alt-tab is released within some time-frame
* Backward cycle using shift
* Intuitive order, respecting your client history
* Includes minimized clients (in contrast to some of the default window-switching utilies)

INSTALLATION:
1)
  cd ~/.config/awesome
  git clone https://github.com/jorenheit/awesome_alttab.git alttab

2) Import into rc.lua:
   local alttab = require("alttab")

3) Optionally edit any subset of the following settings, the defaults are:

   alttab.settings.preview_box = true,                 -- display preview-box
   alttab.settings.preview_box_bg = "#ddddddaa",       -- background color
   alttab.settings.preview_box_border = "#22222200",   -- border-color
   alttab.settings.preview_box_fps = 30,               -- refresh framerate
   alttab.settings.preview_box_delay = 150,            -- delay in ms

   alttab.settings.client_opacity = false,             -- set opacity for unselected clients
   alttab.settings.client_opacity_value = 0.5,         -- alpha-value
   alttab.settings.client_opacity_delay = 150,         -- delay in ms

4) Add key-bindings:
   On my particular system, and I guess most, Shift-Tab is captured by the keygrabber as a 
   single key, namely "ISO_LEFT_TAB". Therefore, this is what my keybindings look like:

   awful.key({ "Mod1",           }, "Tab",
       function ()
           alttab.switch(1, "Alt_L", "Tab", "ISO_Left_Tab")
       end	     
   ),
 
   awful.key({ "Mod1", "Shift"   }, "Tab",
       function ()
           alttab.switch(-1, "Alt_L", "Tab", "ISO_Left_Tab")
       end
   ),