Support 'real' theming

Currently Ember Paper allows developers to override some default colors, like the primary and warn colors. However, there are a few extension points that would be nice to have. This issue is basically here to track the idea and discussion.
##### 1. EP could expose a `theme` service that allows users to control the theme's colors at runtime.

This would be useful to, for example, switch between dark and light themes, or have a different color scheme for different parts of an app.
##### 2. Components could expose a `theme` or `color` attribute that allows local overriding of colors

For example, when using `{{#paper-button color='indigo'`, the button's color would be indigo. This allows for greater control outside of the existing `primary=true`, `accent=true` and `warn=true`. When used on something like `paper-card`, the card's content should adapt to still be readable.