Add keyboard shortcut for opening original page

Closes #510.

assets/javascripts/app/shortcuts.coffee

@@ -137,6 +137,9 @@ class app.Shortcuts
       when 71
         @trigger 'altG'
         false
+      when 79
+        @trigger 'altO'
+        false
       when 82
         @trigger 'altR'
         false

assets/javascripts/templates/pages/help_tmpl.coffee

@@ -90,6 +90,9 @@ app.templates.helpPage = """
     <dt class="_shortcuts-dt">
       <code class="_shortcut-code">alt + r</code>
     <dd class="_shortcuts-dd">Reveal current page in sidebar
+    <dt class="_shortcuts-dt">
+      <code class="_shortcut-code">alt + o</code>
+    <dd class="_shortcuts-dd">Open original page
     <dt class="_shortcuts-dt">
       <code class="_shortcut-code">alt + g</code>
     <dd class="_shortcuts-dd">Search on Google

assets/javascripts/views/content/entry_page.coffee

@@ -5,6 +5,9 @@ class app.views.EntryPage extends app.View
   @events:
     click: 'onClick'
 
+  @shortcuts:
+    altO: 'onAltO'
+
   @routes:
     before: 'beforeRoute'
 
@@ -146,3 +149,8 @@ class app.views.EntryPage extends app.View
       target.classList.add if $.copyToClipboard(target.parentNode.textContent) then '_pre-clip-success' else '_pre-clip-error'
       setTimeout (-> target.className = '_pre-clip'), 2000
     return
+
+  onAltO: =>
+    return unless link = @find('._attribution:last-child ._attribution-link')
+    $.popup(link.href + location.hash)
+    return