Create folders for various CSS types and implement BEM syntax

I think it might be worthwhile to classify our CSS so we know what "type" of style it is.

1. **_global**: For global variables, useful mixins, etc
2. **_lib**: For third-party styles. We may abandon this if we integrate Bower into the mix
3. **_modules**: For the majority of our CSS we can classify everything as a separate module for easy modification. Things like `_accordion.scss`, `_header.scss`, `_navigation.scss`, would fit here. Since we `@import` in the main `.scss` document, it would be ideal to split it all up for easy location.

Sass 3.3+ has great support for BEM syntax. So instead of something like:
```scss
.search-form {
    .search-form__submit {
    }
}
```

We can write it out as 
```scss
.search-form {
    &__submit {
    }
}
```