Bitcoin.pdf: Create Page Linking To Translations

- Adds a new page linking to all translations of Nakamoto's Bitcoin
  paper, and also provides instructions for adding new translations

- Changes all links on the site that used to point directly to the
  English bitcoin.pdf to point to the new page.

- Add Spanish Translation by Breathingdog, as well as Spanish strings
  for the now page also by Breathingdog

_plugins/translate.rb

@@ -87,6 +87,12 @@ module Jekyll
           text.gsub!("#"+key+"#",'/'+lang+'/'+CGI::escape(value))
         end
       end
+
+      ## Hack for renaming links to the Bitcoin paper. Safe to remove
+      ## when all languages have "bitcoin-paper:" defined in the "url:"
+      ## section of their '_translations' YAML file.
+      text.gsub!('#bitcoin-paper#','/bitcoin.pdf')
+
       anc = site['loc'][lang]['anchor']
       anc.each do |page,anch|
         anch.each do |key,value|