Creating a function that we can use to convert px to rem.

This improvement prevent the support of old browsers to REM.
When use font-size or any other property of CSS, you put in value the
*remtopx(rem value)*. The example:

p
   font-size: remtopx(1)
—
p
  font-size: 16px
converting all REM value font-size to *remtopx* function.
Merge pull request #1006 from diegoeis/3.1.0

Convertendo rem para px