Correcting push/pull classes in demo template
Switching from padding to margin for leading classes

Fixes an issue where background images applied to columns using a leading
class extended beyond the bounds of the column into the leading area.
Hiding horizontal overflow of columns

Fixes an issue where non-wrapping text extended beyond the horizontal
boundary of a column if it was wider than the width of the column.
Positioning columns relative by default

Helps with absolutely positioning elements within a column.