Commit Twenty Thirteen Companion styles

This commits initial pass for twentythirteen scss sheet based on the previous work for 2014/15.

• Variables, Mixins adjusted for 2013 colors and specific styling.
• Breakpoints adjusted for BP elements to suit 2013 layout.
• BP navigational elements and lists tweaked to suit layout.

This provides a good base now for iterating further styles on.

See #6533