Fix `Value Error : font-size Unknown dimension 1pxrem`

Strip the unit from @base-font-size-sm
.rem() expects a unit-less value, which it believes is a px value.
Fix Value Error : background-image bottom is not a color value )

Use a valid angle keyword

http://dev.w3.org/csswg/css-images-3/#linear-gradient-syntax
Strip the unit off of variables in the .rem() mixin
use break-inside instead of column-break-inside

The property is break-inside.  http://www.quirksmode.org/css/columns/breaks.html
Merge pull request #655 from mfairchild365/css-validation

Css validation fixes