Support script's metadata better

I think you should extract more information from metadata: license (you can use tldrlegal.com), screenshot url, icon, permissions, etc ...

I think that icon and screenshot are the most important ones to extract and display.
And fix @contributionURL and @contributionAmount display : show them on main page in form of nice button (you can see an example if you install Scriptish and open the script's page in about:addons), not bad-looking link as it is implemented now.