Make font sizes relative

I've found that the font size on iOS is too small to read the comments comfortably. I've basically made two changes:

1. I switched everything over to rem units which means that everything displays exactly the same, but will respect the default font size of your iOS device.

2. I made the comments 1.6rem, i.e., 100% of the default font size, which I think makes sense for "body text" on a mobile site. (Setting this to 1.3rem would result in exactly the same display behavior as before under.)