_s: Simplify author template.

Set up author data for the author template immediately, rather than
waiting for the first the_post() call.
This removes the need to call the_post() and rewind_posts() in an
author template to print information about the author.

Fixes #346.
Merge branch 'master' of git://github.com/Automattic/_s into upstream
Merge branch 'upstream'
Merge branch 'master' into customized