Writing content /install

- Restructured the content to focus on the 2 ways to get Sass installed: GUI & the command line
- Added icons (they need to be converted to SVG or webfonts so they look pretty)
- Added a version YML file so we can add the current Sass version, name and release notes in a single location to use throughout the site.
Updated the version name / number throughout the site.
Undoing codeblock tinkering.
Merge branch 'master' of github.com:sass/sass.github.com