added variables && adjusted headings to react to variables

added line-height helper class

added documentation to line-height

fixed line-height mixin

added line-height mixin to headings

replaced all font-size: 16 with base font size variable

imported variables into text-margin

changed name from line-height to m-line-height

using base-font size & base-line-height from the settings file now.
Merge pull request #52 from yoshuawuyts/rem-typography

Added variables to font size, line height ratio and added responsive line heights