Improve manifest and add some icons

I've fixed some issues in the manifest (see 492e330) and added extension icons to make more "well developed". 
Icons are surely not perfect, but I think it's better than nothing.
Also the source PSD is included.

:sunflower: 
> Unfortunately I cannot change the name anymore.

Why?



> to trademark issues

You really think so?
https://chrome.google.com/webstore/search/github?_category=extensions

Your are also explicitly mentioned as the author.