Add disabled button style

Really simple base styles - just reset the cursor. Trying to avoid
including too much opinionated style in this template by default.

Fix gh-3
Include `is-disabled` in README

Also slightly reorganize and flesh out some of the README content.
Remove small and large modifiers

These modifiers are the responsibility of a theme component. Theme
authors shouldn't have to override the default sizes, which is likely to
happen.
Update README to reflect modifier changes
Update CHANGELOG
Add ignores to `bower.json`
Use inert `href` values for test anchors
Prevent user text selection for buttons

Avoid the text in buttons being selectable so it doesn't get copied to
the clickboard, or receive text-selection styles in the UI.
Improve the README section on HTML classes