Adds global styles for layout and icons

[HaSistrunk]

Fixes #54 and #53

• Adds Neat (this is preliminary, and will need to be reviewed as part of issue #4 after we add component styles).
• Adds global styles for CSS helper classes including .container, .container--full-width, .wrapper, and .visually-hidden.
• Adds gutters variable and replaces boilerplate breakpoints with breakpoints used in dimes.
• Imports Material Icons font and adds an icon component with associated styles.