Adds button styles

[helrond]

Adds styles for buttons. fixes #3

There are also a few other things in this PR:

• I had to make a bunch of changes to the stories to add classes, etc.
• I've introduced two spacing classes for material icons
• I've renamed the material-icons class to material-icon, which is more semantically correct. This has been documented in the change tracking document.
• Note that btn--orange currently does not meet the WCAG contrast threshold. This has been documented in the Known Issues section.

[HaSistrunk]

Yeah, this all looks good to me and the comments are a useful addition. I see that btn--transparent is the only one without hover/focus styles (beyond the global focus styles that are defined outside of the component). This made me think about two things:

1. Perhaps we should change the border color on hover/focus for the transparent button with a darken calculation that mirrors other button styles and gives some indication to the user.
2. Meeting accessibility requirements in terms of color contrast for the transparent button will depend on the color of whatever is behind the button on the page. So when using that button, the developer should check that the button text color and the page background have sufficient contrast. Perhaps we could include a note about this in the documentation, maybe in the best practices section?