Converted all font-size to rem units

This allows implementers to override all font sizes without the need to recompile and still be able to use the code from CDN. Implementers only need to add `body{font-size:...}` to their style sheets.

This also allows implementers that need to conform to WCAG 2.0 to comply with the previous line of code (it is a failure of WCAG 2.0 to use absolute units such as pixels).