Edited PS description, added buttons to home page

Hey Shubie,

I changed the PS description to exclude the funding restriction (Brandon and I previously decided not to limit teams based on funding).  I also added buttons to the home page so people who visit bases.stanford.edu/challenge have a direct link to more info for each of the competitions.  Apologies for the hackiness of the CSS - feel free to change it however you see fit :)

Daryl