Paper-button doesn't use a proper font style

[jhrdina]

I feel like paper-button should use paper-styles/typography.html and its --paper-font-button mixin for font.

Expected outcome

Paper button uses 14px font with font-weight: 500 (Roboto Medium) as mentioned in Google Design Spec's Typography section and is already defined in paper-styles/typography.html.

Image: material-design-lite demo, their CSS implementation here

Actual outcome

Paper button uses default font size with default font-weight (400 - Roboto Regular).

Image: paper-card demo

[abdonrd]

👍

[abdonrd]

PR about this: #96

[blasten]

cc @notwaldorf

[keanulee]

We decided not to do this for a11y reasons - see #95 (comment)