Susy and compass

This PR contains a tutorial sketch with complete set of screenshots: _Using Susy and Responsive Grids sample from [NET TUTS+](http://net.tutsplus.com/tutorials/html-css-techniques/responsive-grids-with-susy/)_.

Some of screenshots need editing - adding arrows/highlighting to point on screenshots areas, where the main action(s) presented by that screenshots are originated from/are happening at.

Tutorial sketch text has to be verified, and, when needed, augmented with additional information and with references to the other parts of the [Prepros App online documentation](http://alphapixels.com/prepros/docs/index.html).