Added Compass vendor prefixes functions

Noticed those were missing when trying to:
````  @include background(linear-gradient(left top, white, crimson));````

They work very well now for -webkit- and -moz- but for some reason the others are not behaving.
The code is taken from the [php-compass](https://github.com/tsi/phamlp/blob/master/sass/extensions/compass/functions/crossBrowserSupport.php) version I used with Phamlp where they somehow worked without issues, I'm probably missing something here, but I must leave this for the moment, so figured I better push this as is.