add role="tooltip" to tooltips by default to improve accessibility

See https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#tooltip

js/tooltip.js

@@ -29,7 +29,7 @@
     animation: true,
     placement: 'top',
     selector: false,
-    template: '<div class="tooltip"><div class="tooltip-arrow"></div><div class="tooltip-inner"></div></div>',
+    template: '<div class="tooltip" role="tooltip"><div class="tooltip-arrow"></div><div class="tooltip-inner"></div></div>',
     trigger: 'hover focus',
     title: '',
     delay: 0,