Responsive makeColumn/makeRow and use global variables

Created by: barryvdh

• Create 2 different makeColumn/makeRow mixins with guard expressions.
• Make parameters optional in the grid() mixin (default to global variables)
• Override grid variables in responsive.less, to make makeColumn work

This enables creating 1 mixin with .makeColumn and .makeRow, and have the possibility to collapse, just like the regular spans (or define columns/rows in mediaqueries)

@media (max-width: 767px) {
    // Custom grid
    @gridColumnWidth: auto;
    @gridGutterWidth: 0;

    #custom > .grid();
}

(For issue #2242 (closed) #2448 (closed) and #1636 (closed)