Fixes #15, changes timestamps slightly

This adds dates to petition cards, and changes very slightly how dates are shown on the single petition pages.

This adds a global handlebars relativeDate handler, for use with moment. This also changes the petition page to use that helper.

This implementation is similar to Facebook's relative date formats. It's nice because it displays the relative time which is enough in most cases, but the relative time's title is the actual date and time it was posted, for reference. I'll be honest and say that I added the `data-utime="xxx"` attribute in the `<abbr>` tag because Facebook does it, but it's not strictly necessary for this implementation, and could be removed.

Please let me know if this could be improved.