Applying several tweaks

My changes are mostly related to I18n. Basically I changed the wording of the system messages and hope that you are ok with it. I also expanded the README. Thank you for creating this extension.

Another story: What about renaming the extension to something like "Disallow-CSS-JS-Import" or so ... Thus it is easier to grasp the functionality a bit better. Will probably be a better "marketing name", too.