Add COinS span to posts.

[jeremyboggs]

See @walshbr's example on his site:

<span class="Z3988" title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Adc&amp;rft.title={{ page.title | cgi_escape }}&amp;rft.aulast={{ site.contact.name_last | cgi_escape }}&amp;rft.aufirst={{ site.contact.name_first | cgi_escape }}&amp;rft.source={{ site.title | cgi_escape }}&amp;rft.date={{ page.date | date_to_xmlschema }}&amp;rft.type=blogPost&amp;rft.format=text&amp;rft.identifier={{ site.url | cgi_escape }}{{ page.url | cgi_escape }}&amp;rft.language=English"></span>