add options other than 'fork me on github' as images for FB and G+ posts

multiple itemprop images should be valid according to http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#the-basic-syntax

additionally, I am not sure if we are allowed to use the ACM logo.  I assume since we are affiliated with the ACM (however loosely), it would be ok.