Add rem unit mixin for font-size

This mixin allows easier use of `rem` units when working with font-size. Rem gives user resizable text, just like em units, without the hassle of compounding font-sizes. More info about the advantages of using `rem` over `em` or `px` can be found here: http://snook.ca/archives/html_and_css/font-size-with-rem

As `rem` is not supported by IE8 and lower (http://caniuse.com/#search=rem), we still need to provide a pixel fallback, hence the need for a mixin. The easiest way to work with this is to set `font-size: 62.5%` on the `html` or `body` tag, wich gives us a nice `1rem == 10px` relation to work with.

The usage is:

``` scss
h2 {
  @include rem(18);
}
```

wich outputs:

``` css
h2 {
  font-size: 18px;
  font-size: 1.8rem;
}
```

Let me know what you think!