Add "Semantic grid" capabilities

Complete list of changes:
- Change title from atGrid to atGrid.css
- Gutter from 20px to 30px
- Add "only-semantic" option
- Add at-container() to create containers
- Add at-row() to create rows
- Add at-column() to create columns
- Change style of comments
- Add contact email to license file
- Update package name from "atgrid" to "atgrid.css"
- Update description in package.json
- Add contact email in package.json
- Update devDependencies in package.json