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: 
"Please do not use trademarked material or use words/logo that may see you are associated with GitHub when you are not " (tl;dr). This is standard Counsel reply on trademark issues in the US. Trademark law is a wonderful world of 1000 things you need to do in order to keep it.