add @total-width as parameter to .column and .row

To allow mixed pixel and percentage layouts.
Add variable for containing element

Useful if using within other templates
Comment formatting
add @total-width parameter to SCSS and Stylus
Add helper mixing for responsive elements

As a shortcut, instead of column(x,@totalwidth: 100%), you can just use
column-resp(x) or row-resp(x)
document modifications