Use baseline for vertical spacing and gutter for horizontal spacing

- `$gutters` convey horizontality
- `$baseline` conveys verticality

We should use them accordingly. This PR aims to normalize their usages where it made sense.

PS: PR contains some small bits of refactoring where I saw values that could be simplified.


![oh yeah](http://i.minus.com/ibhAIGpjv4fWZg.gif)