Redesign landing page, add new pages

This branch satisfies a few Pivotal stories around our landing website.

#### Make landing actionable -- add Mailchimp form (607cd6a)

Previously our buttons told the user to get started with the app. We won't be ready to launch for a bit, so I changed them to ask users to sign up for our Mailchimp list and get updates on the app.

#### Add topper section with image (cf14d66 and cf14d66)

I redesigned the top of each page to add an image. I think this creates more context for what the app does, and takes the burden off of us to keep finding interesting icons/illos for each page. I think it also gives the pages a bit of a 'glossy' feel.

#### Create about.html (4d95aac and 4d95aac)

Satisfies Pivotal story describing what the application does. Could use some work down the line to tweak language and determine what the common questions we'll need to answer are.

#### Create basic contact.html page (8d369fd)

Right now contact.html is just an `a href=mailto` link. Down the line, we can form-ify this, but this version works.