Reworking site template to utilize Sass

Alright @parkr, there it is. Got a bigger than I thought it would.

So folks, apologies for any mistakes I might have made. This is my first bigger pull request touching more than 2 lines of code at a time.

I'm open for questions/suggestions and won't be mad if this doesn't get merged. :)

## Touched files:

in `/lib/site_template`:

```
.
├── _includes:
|   ├── header.html
|   ├── footer.html
|   └── head.html
├── _layouts:
|   ├── default.html
|   ├── page.html
|   └── post.html
├── _posts:
|   └── 0000-00-00-welcome-to-jekyll.markdown.erb
├── _sass:
|   ├── _base.scss
|   ├── _typography.scss
|   ├── _site-header.scss
|   ├── _site-footer.scss
|   ├── _page-content.scss
|   ├── _posts.scss
|   └── _syntax-highlighting.scss
├── css
|   └── main.scss
└── index.html
```

## General

- Reworded the _Welcome to Jekyll_ post a bit. (plus correct apostrophes, yay: `’` instead of `'`)
- Small changes to breakpoints (750px break point is now 800px, matching `.wrapper`)

## HTML/Markup changes

- Making `site-title` a proper heading (`h1`)
- Adjusting document outlines accordingly (post and page headings are now `h2`’s)
- Changes to `head.html` just change order of the items so things are distinctable more easily.

## CSS/Styles changes

- Use `box-sizing: border-box;` globally
- Using variables for fonts, colors and unified spacing
- __Code formatting__
    - Removing `.terminal` styles because they weren’t used on the site
    - Moving code styles to general typography (they were nested inside `.post` before, but might as well appear outside of posts)
    - Removed prefixed property declarations for `border-radius`, because  most browsers support the unprefixed version nowadays
    - Set overflow scrollbars for codeblocks only on horizontally, not vertically
- __Site header__
    - Removed unnecessary `background-color`
    - Cleaning up some of the styles for the navigation
    - Removing unnecessary HTML attributes from the `svg` element (moving fill color to CSS)
- __Site footer__
    - Using `icon` class for general icon styling
    - Removing unnecessary HTML attributes from the `svg` elements (moving fill color to CSS)
    - Simplifying the little _grid_ in general
    - Removing widths with `calc` which isn’t needed with the alternate box model
I thought about adding something like this, i.e. something inspired by the media query mixins Harry Roberts utilizes in his inuitcss (great stuff).

I'm about concerned about the added complexity, though. We want to keep it simple for new users. What do you think?