Brings single author links into hcard compliance

Google now looks for structured data on pages (lacking that data is
considered an error by Webmaster Tools). These changes bring
co-authors-plus's output in-line with the hatom hcard microformat for
authorship.
Providing arguments for optional span class

As automatically adding the span class might break existing themes,
moved this implementation to make it optional and applied through
filter, while retaining the in-link class.

I'm unsure of coding standards for this setup, but I figured that the
bracketing variables should be added after the sprintf and it made sense
to break down the return into two steps. Open to a better way of doing
this if there is one.
Merge pull request #160 from AramZS/master

Brings single author links into hCard compliance