Added "Fork me on Github" ribbon.

Styles will follow, but this is the image and the link.
Added styles for "Fork me on Github" ribbon.
Update main.less
fixed compile error due to non-defined variable
fixed not displayed and that it's an id instead of a class.
css did not build automatically
Merge pull request #86 from cimnine/patch-1

Add "fork me on github", but only on desktop devices