Refactor kind designation for components to use an enum

[jtrussell]

This one has bugged me for a while. I think it's only the way it is currently because it was easier (?) to generate our atoms programmatically.

But consider an API change from:

<AuButton primary>
  Clicky
</AuButton>

to:

<AuButton kind="primary">
  Clicky
</AuButton>

Why? It makes it impossible to do something like this:

<AuButton primary secondary>
  Clicky
</AuButton>