Home page clean up

@laspluviosillas @davidclarkmoore  Here are a few tweaks to the home page. I didn't bother tweaking the images that are sized improperly because we haven't decided on the actual images that will go on the home page yet. @laspluviosillas I wasn't sure how you wanted to change the mission stories or which right arrows you were referring to in the "Share this!" text. Is it supposed to be a button?

@davidclarkmoore - I didn't see your previous branch by the same name (except with "Home" capitalized) so my apologies for any confusion. It appears your branch is pulled into the master so I'm guessing we can kill it. 
Incorrect level of nesting here. `li {` should be in line with all the other properties.