Add support for Co-Authors Plus

Adds authors generated by the Co-Authors Plus plugin (supported by WordPress.com VIP) to the heading and fixes a missing "%s" in the heading for regular author pages without the plugin
Add support for Co-Authors Plus

Adds support to the byline for authors generated by the Co-Authors Plus plugin (supported by WordPress.com VIP) and simplifies the regular author link that is displayed when you don't have the plugin installed
Make author heading code more readable
Make byline code more readable
Get rid of that semicolon
Use a hook instead of the if structure
Use a hook instead of the if structure
Add back the rest of the byline
Put the byline back in there, again
Merge pull request #825 from jonatansteller/master

Add Co-Authors Plus plugin compatibility to the mobile theme