wibox: Add more constructor arguments

Fixes #916

lib/awful/wibar.lua

@@ -307,6 +307,8 @@ function awfulwibar.new(arg)
         end
     end
 
+    arg.screen = nil
+
     local w = wibox(arg)
 
     w.screen   = screen

lib/wibox/init.lua

@@ -185,6 +185,19 @@ local function new(args)
         end
     })
 
+    -- Set other wibox specific arguments
+    if args.bgimage then
+        ret:set_bgimage( args.bgimage )
+    end
+
+    if args.widget then
+        ret:set_widget ( args.widget  )
+    end
+
+    if args.screen then
+        ret:set_screen ( args.screen  )
+    end
+
     return ret
 end