feat(button): disabled attr on button css styling

ionic/components/app/normalize.scss

@@ -127,7 +127,8 @@ button {
 }
 
 // Re-set default cursor for disabled elements.
-.button-disabled,
+a[disabled],
+button[disabled]
 html input[disabled] {
   cursor: default;
 }

ionic/components/button/button.scss

@@ -35,7 +35,7 @@ a.button {
   text-decoration: none;
 }
 
-.button-disabled {
+.button[disabled] {
   opacity: 0.4;
   cursor: default !important;
   pointer-events: none;

ionic/components/button/test/basic/main.html

@@ -35,4 +35,8 @@
     <button dark class="activated">Dark.activated</button>
   </p>
 
+  <p>
+    <button disabled>Disabled</button>
+  </p>
+
 </ion-content>