Added mobile- and push classes.

Hey,
i added some mobile- and push classes to the CSS File and added a little introduction in the Readme.

The mobile classes are very useful and i don't want to miss them in all of my projects.
As a example: Display 5 images in a row on desktop, and 3 on mobile.

The push-classes are copied from another fork.

fyi: Why you use the ":last-of-type" selectors? I have delete the two rules and it works on all devices.