Improved position add-on v.2

Sorry for the duplication (from [this pull request](https://github.com/thoughtbot/bourbon/pull/226)), but I had a change of heart. I don't think the named coordinate variables are necessary, given that using them is not much different from just entering a CSS rule. Also, the ability to force an auto value might be handy. So here's what the changes I'm submitting should now amount to:
- Unitless zeroes accepted.
- `n` is a neutral/null value that can be plugged into the coordinate list to "skip" that value. For example, `n n 10px 5px` will only output the rules `bottom: 10px; left: 5px`.
- `a` is a shortcut for `auto`.