Avoid throwing error when filtering undefined elements

When a site uses [`jQuery.append`](http://api.jquery.com/append/) to insert HTML that contains emoji, the script will fail and kill the function that called `.append`. (Strangely, the same does not happen for `jQuery.prepend`.)

This pull request:

1. verifies that `.just_text` is an acceptable method before moving on
1. caches `$(this)` for better performance (regardless of this bug)

![image](https://f.cloud.github.com/assets/262137/2066330/834ceeaa-8ced-11e3-9d09-88e81c79d3d9.png)