docs(plugins): add import statements for usage clarity

src/plugins/imagepicker.ts

@@ -27,6 +27,10 @@ export interface ImagePickerOptions {
  *
  * @usage
  * ```js
+ * import {ImagePicker} from 'ionic-native';
+ *
+ *
+ *
  * ImagePicker.getPictures(options).then((results) => {
  *   for (var i = 0; i < results.length; i++) {
  *       console.log('Image URI: ' + results[i]);