Add New Glossary Page And Developer Search Feature

This commit adds the code necessary to generate a new
en/developre-glossary page with entry pages in the en/glossary/
directory, e.g. en/glossary/51-percent-attack.

The glossary page and the individual term pages feature a JavaScript
search engine (no CGI) for just glossary terms.  This search box has
also been added to the following pages:

    * Developer Documentation (the index page)
    * Developer Guide
    * Developer Reference
    * Developer Examples

The search box requires the following MIT-licensed libraries:

    * JQuery
    * JQuery UI
    * JQuery UI CSS stylesheet

These allow our JS code to be almost trivially simple in js/devsearch.js

This commit adds only code.  Actual glossary entry data will be added in
a subsequent commit.

_plugins/autocrossref.rb

@@ -35,12 +35,37 @@ require 'yaml'
 
       ## Workaround for inconsistent relative directory
       path = File.expand_path(File.dirname(__FILE__)) + "/.."
-      ## Load terms from file
+      ## Load terms from file only if we haven't loaded them before
       site = context.registers[:site].config
-      if !site.has_key?("crossref")
-        site['crossref'] = YAML.load_file(path + "/_autocrossref.yaml")
+      if !site.has_key?("crossref_loaded")
+
+        ## Load refs from file and then downcase them all so we can
+        ## easily detect when we define xrefs more than once
+        mixed_case_refs = YAML.load_file(path + "/_autocrossref.yaml")
+        unvalidated_refs = Hash.new
+        mixed_case_refs.each { |key, value|
+          unvalidated_refs[key.to_s.downcase] = value.to_s.downcase
+        }
+
+        if site.has_key?("crossref")
+          ## We already have refs loaded, so merge
+          site['crossref'].merge!(unvalidated_refs) {
+            |key, old_value, new_value|
+
+            if old_value != new_value
+              abort("Error: autocrossref key '#{key}' wants to point to both '#{old_value}' and '#{new_value}'")
+            end
+
+            new_value
+          }
+        else
+          ## We don't have refs loaded yet, so copy
+          site['crossref'] = unvalidated_refs
+        end
+        site['crossref_loaded'] = true
       end
 
+
       ## Sort terms by reverse length, so longest matches get linked
       ## first (e.g. "block chain" before "block"). Otherwise short
       ## terms would get linked first and there'd be nothing for long
@@ -76,7 +101,7 @@ require 'yaml'
             (?!\w)  ## Don't match inside words
             (?!`)   ## Don't match strings ending with a tic, unless the xref itself ends with a tic
           /xmi) {|s|
-              if term[1] == "DO NOT AUTOCROSSREF"
+              if term[1] == "do not autocrossref"
                   s.gsub(/( |$)/, "<!--noref-->\\&")
               else
                   "[#{s}][#{term[1]}]{:.auto-link}"