Add $v-space and $h-space variables instead of $margin

Use $v-space and $h-space variables instead of $margin (Fixes #10)

Status: **Ready for speedy merging**
Reviewers: @jeffkamo @ry5n 

## Changes
1. Removed `$margin` variable
1. Added `$v-space` and `$h-space` references
1. Changed out all references to `$margin` with references to `$v-space` or `$h-space` depending on the context.
1. Fixed an issue with `fieldset` needing `min-width` to be reset to `0`.

## Notes
- Funny issue, the `min-space` one. It seems like Safari defaults to `-webkit-min-content` as a value for `fieldset`. This causes the `fieldset` to be quite a bit wider than you may want in some situations. Resetting `min-width` to `0` fixes this.
- Fixes #10.