Internationalized

Neat plugin. I've just included it in a project; however, I required multi-language. 

Thus, I've internationalized the plugin (*a simple contribution*) -- though there is still the need to create language files for other languages (*I only included english, french and spanish*).

**Note:** CKEditor was attempting to pull the language file *lineheight/lang/lang_code.js* for this plugin; which would 404; this merge request obviously resolves this issue as well.