fix(alert): add missing cssClass property for buttons

Commit 46fe1ff defined the `AlertButton` interface for the `buttons` array, but it's missing the `cssClass` property as described in the **Button options** section of the docs: http://ionicframework.com/docs/v2/api/components/alert/AlertController/#advanced

Author: FdezRomero

src/components/alert/alert-options.ts

@@ -27,5 +27,6 @@ export interface AlertInputOptions {
 export interface AlertButton {
   text?: string;
   role?: string;
+  cssClass?: string;
   handler?: Function;
 };