Update plugin.js

as it is written here , http://www.tinymce.com/wiki.php/Configuration:remove_script_host
`If disabled, URLs will be returned in this format: "http://www.example.com/somedir/somefile.htm" instead of the default format: "/somedir/somefile.htm".`

now, even if I set
```
relative_urls: false,
remove_script_host: true,
convert_urls: false,
```
it still appends the base url(so it is not being possible to leave the image url intact), but checking the condition that was added, if that option is false only in that case base url is being appended.