Twenty Fourteen: add "button" CSS class for styling elements as buttons

[celloexpressions]

It is often desirable to style links or other elements as buttons for various purposes. In-content this allows very important (user-flow-type) links to gain emphasis (ex. "become a member" at the end of an about page). In plugins, it allows buttons that aren't technically buttons to inherit the theme's styling instead of creating their own, duplicating the theme's styling on a custom id (ex. Jetpack's infinite scroll), or using hacks to use the button-type elements that are styled by the theme.

In Twenty Fourteen itself, .contributor-posts-link would benefit from using .button for its generic button styling, plus the more specific class for styling specific to that particular button.

The use of the .button class in wp-admin provides inspiration for this proposal, as it has use cases on the front-end as well.

[celloexpressions]

Add a .button class

[celloexpressions]

Add a .button class and use it for the contributor posts link.

[lancewillett]

+1 Let's try to get this in for 3.9 cycle.

[nacin]

In 27442:

Twenty Fourteen: Add a .button class.

props celloexpressions.
fixes #27075.