Improve position mixin

It works like if you use margin. It has support for shorthands and has null property to avoid values if you only wanna set some of the values.

Squash at will
Actually i didn't know that there was an unpack mixin

But i do believe that i makes a good mixin for the position in this case, so it works like margin og padding.

Anyways i don't know why you have the unpack function on a margin that's standard behavior isn't it? At least down to IE6 i used it their i remember.