fix(dropdown): Emit shown and hidden events (issue #757)

Author: tmorehouse

lib/mixins/dropdown.js

@@ -68,6 +68,7 @@ export default {
 
             if (state) {
                 this.emitOnRoot('shown::dropdown', this);
+                this.$emit('shown');
                 /*
                  If this is a touch-enabled device we add extra
                  empty mouseover listeners to the body's immediate children;
@@ -82,6 +83,7 @@ export default {
                 }
             } else {
                 this.emitOnRoot('hidden::dropdown', this);
+                this.$emit('hidden');
                 /*
                  If this is a touch-enabled device we remove the extra
                  empty mouseover listeners we added for iOS support