Added Compass' Vertical Rhythm module

I've ported this over to Stylus from Compass. It works mostly like before, except all variables don't have dollar signs except for the `$rhythm()` function. That had to have a dollar sign because of the `rhythm()` mixin that shares the same name in the original project, as Stylus does not hold separate scopes/data types for mixins and functions like SASS does.

On top of those differences is the following:
1. Any deprecated features of the original Compass module have been removed.
2. I left out the `h-borders()` alias. Use `horizontal-borders()` instead.
3. A `unitless()` function was added to `utilities.styl` that mirrors the function of the same name in SASS.
4. `debug-vertical-alignment()` differs greatly. It's a temporary solution, it takes no parameters and defaults to fetching an image from an online service at http://basehold.it