Refactor menu, fix ripples on disabled menu items

Fix ripples / onSelect engaging on disabled menu items.

Now maps over offsetTops, offsetHeights rather than continuously
looking up in Arrays. Avoids conversion List -> Array on Open event.

Fix #142.

demo/Demo.elm

@@ -387,7 +387,7 @@ main =
     , view = view
     , subscriptions = \model ->
         Sub.batch
-        [ Sub.map MenusMsg (Menu.subs Demo.Menus.MDL model.menus.mdl)
+        [ Sub.map MenusMsg (Menu.subs Demo.Menus.Mdl model.menus.mdl)
         , Layout.subs Mdl model.mdl
         ]
     , update = update
@@ -425,7 +425,6 @@ stylesheet =
     display: inline-block;
     box-sizing: border-box;
     min-width: 100%;
-    background-color: #f8f8f8;
     padding-top: .5rem;
     padding-bottom: 1rem;
     padding-left:1rem;

demo/Demo/Code.elm

@@ -9,6 +9,7 @@ import Markdown
 import Material.Elevation as Elevation
 import Material.Options as Options exposing (css, div, stylesheet, Property)
 import Material.Helpers as Helpers exposing (cmd)
+import Material.Color as Color
 
 
 type State 
@@ -133,6 +134,7 @@ code options str =
       [ css "overflow" "auto" 
       , css "border-radius" "2px"
       , css "font-size" "10pt"
+      , Color.background (Color.color Color.BlueGrey Color.S50)
       , Elevation.e2
       ] :: options)
     [ Markdown.toHtml [] <| "```elm\n" ++ trim str ++ "\n```" ]