Request smaller sized images from GitHub

By default the GitHub API serves the avatars as `460x460px` images.
Since we only display the avatars as `25x25px` images, we're:

   * forcing the browser to get way more data then it actually needs
   * wasting the CPU (resizing those large images consumes CPU)

This commit fixes the issue by making the page request smaller sized
images (done by adding the `s` field with the value of the image size
to the query string), while also ensuring that those images look good
on different resolution screens.

Ref h5bp/h5bp.github.io#16
    h5bp/h5bp.github.io#15

Close h5bp/h5bp.github.io#16