Add OpenGraph to each page (#147)

OpenGraph Protocol is used by sites like Facebook to display a rich object in a social graph.

* Add OpenGraph Protocol to each page

Hugo has an internal template for [Open Graph](https://gohugo.io/templates/internal/#open-graph). It takes necessary information from either site's or post's config thus it makes sense to add it to each page.

* Add 'series' to posts archetype
This taxonomy is used to specify related “see also” pages by placing them in the same series.

* Add series taxonomy to exampleSite config.

* Add example of series to exampleSite

* Add spaz926 to Contributors

layouts/_default/baseof.html

@@ -10,6 +10,7 @@
     {{ with .Site.Params.keywords }}<meta name="keywords" content="{{ . }}">{{ end }}
 
     {{ template "_internal/twitter_cards.html" . }}
+    {{ template "_internal/opengraph.html" . }}
 
     <base href="{{ .Permalink }}">
     <title>{{ block "title" . }}{{ .Site.Title }}{{ end }}</title>