Updating the plugin with some changes

Hello Chris,
I made some changes you may want to consider moving into the repo.

* Don't overwrite the discourage search engines flag anymore
* When clearing the content from the *Robots.txt Content* field, you will be prompted to save the reloaded defaults.
* Shows an iFrame of your most current Robots.txt file below the edited content.
* Allows for a moved wp-content folder and a moved media uploads folder now.

I liked what you did here quite a bit.  I have some custom layouts for my wordpress sites like so:
`/wp/  
/wp-content/
/media/`

These changes handle my structure and others also.