Make just-check work for Scottish just-eat

The Scottish FIS has a different syntax in its JSON returns rather than a numeric rating.  This set of changes includes a set of local images and uses them to make the extension work with the Scottish ratings as well as the rest of the UK.

This is my first attempt at using GitHub (as well as Chrome content scripts/extensions), so please let me know if I've got something wrong.

If this goes well, I'm going to try and extend it to work on Hungry House as well :-).