Include debug function and include it in span mixin

Hey Eric, 

Here's one way we can include the debug function. I'm curious to see how it compares to what you were thinking. We probably shouldn't merge this into master yet. I'm just posting it here for an early review.

Please note the to-dos under the debug function. We should consider translating the true / false statements into something more valuable, such as ```(is-nested)``` or something like that. We will also want to get a ```$debug``` option added to ```$susy``` options.

Also, this ```susy-debug()``` mixin can be extended to test for any Susy mixin by adding another ```@if``` statement.

Thoughts?