Enable the no-else-return ESLint rule

Using `else` after `return` is not necessary, and can often lead to unnecessarily cluttered code. By using the `no-else-return` rule in ESLint we can avoid this pattern, see http://eslint.org/docs/rules/no-else-return.

src/display/annotation_layer.js

@@ -81,9 +81,8 @@ AnnotationElementFactory.prototype =
               return new RadioButtonWidgetAnnotationElement(parameters);
             } else if (parameters.data.checkBox) {
               return new CheckboxWidgetAnnotationElement(parameters);
-            } else {
-              warn('Unimplemented button widget annotation: pushbutton');
             }
+            warn('Unimplemented button widget annotation: pushbutton');
             break;
           case 'Ch':
             return new ChoiceWidgetAnnotationElement(parameters);