Adjust buttons component so that it's up to date and accurate

[jenniferthibault]

Filed as a next step to #47

In order to ensure that the pattern library shows the canonical of the accordion component, make
the following changes:

Changes to the basic category:

• Remove active state. The active state will be shown through the toggle component
• Add more spacing between buttons so that they don't look so much like toggles

Changes to the icon category:

• Show each button in just one color—color changes can be determined by the basic tab, and this will cut down clutter
• Add dropdown buttons. These examples were missing from the components

Changes to the sizes category:

• Remove the colored background slab to be consistent