feat(ionic.Platform): add ionic.Platform.setGrade() function

Closes #1104

js/utils/platform.js

@@ -71,7 +71,25 @@
         for(var i = 0; i < ionic.Platform.platforms.length; i++) {
           document.body.classList.add('platform-' + ionic.Platform.platforms[i]);
         }
-        document.body.classList.add('grade-' + ionic.Platform.grade);
+      });
+    },
+
+    /**
+     * @ngdoc method
+     * @name ionic.Platform#setGrade
+     * @description Set the grade of the device: 'a', 'b', or 'c'. 'a' is the best
+     * (most css features enabled), 'c' is the worst.  By default, sets the grade
+     * depending on the current device.
+     * @param {string} grade The new grade to set.
+     */
+    setGrade: function(grade) {
+      var oldGrade = this.grade;
+      this.grade = grade;
+      ionic.requestAnimationFrame(function() {
+        if (oldGrade) {
+          document.body.classList.remove('grade-' + oldGrade);
+        }
+        document.body.classList.add('grade-' + grade);
       });
     },
 
@@ -89,7 +107,7 @@
 
     _checkPlatforms: function(platforms) {
       this.platforms = [];
-      this.grade = 'a';
+      var grade = 'a';
 
       if(this.isWebView()) {
         this.platforms.push('webview');
@@ -115,12 +133,14 @@
           this.platforms.push(platform + v);
 
           if(this.isAndroid() && version < 4.4) {
-            this.grade = (version < 4 ? 'c' : 'b');
+            grade = (version < 4 ? 'c' : 'b');
           } else if(this.isWindowsPhone()) {
-            this.grade = 'b';
+            grade = 'b';
           }
         }
       }
+
+      this.setGrade(grade);
     },
 
     /**