add icon links for twitter, github, and IRC; and use scss, because jekyll allows

just using a bit of scss, but it looks like it can help a lot to ensure consistent colors

also restored helvetica neue is font of choice
link to repo GUI *and* to dir, simplify (a bit) how url's are computed, and tidy guide downloads page
notes on use of rdiscount and pygments (neither helpful though)
remove duplicate jquery js load

this was breaking superfish; but things seemed to be working, so maybe we don't need superfish!