On Windows, use alt modifier instead of ctrl for back/forward navigation

Fixes #51.

assets/javascripts/app/shortcuts.coffee

@@ -2,6 +2,7 @@ class app.Shortcuts
   $.extend @prototype, Events
 
   constructor: ->
+    @isWindows = navigator.platform?.indexOf('Win') >= 0
     @start()
 
   start: ->
@@ -72,12 +73,14 @@ class app.Shortcuts
       when 13
         @trigger 'superEnter'
       when 37
+        unless @isWindows
           @trigger 'superLeft'
           false
       when 38
         @trigger 'home'
         false
       when 39
+        unless @isWindows
           @trigger 'superRight'
           false
       when 40
@@ -97,9 +100,17 @@ class app.Shortcuts
     switch event.which
       when 9
         @trigger 'altRight', event
+      when 37
+        if @isWindows
+          @trigger 'superLeft'
+          false
       when 38
         @trigger 'altUp'
         false
+      when 39
+        if @isWindows
+          @trigger 'superRight'
+          false
       when 40
         @trigger 'altDown'
         false

assets/javascripts/templates/pages/help_tmpl.coffee

@@ -1,4 +1,5 @@
-ctrlKey = if navigator.userAgent.indexOf 'Mac OS X' then 'cmd' else 'ctrl'
+ctrlKey = if navigator.userAgent?.indexOf('Mac') >= 0 then 'cmd' else 'ctrl'
+navKey = if navigator.platform?.indexOf('Win') >= 0 then 'alt' else ctrlKey
 
 app.templates.helpPage = """
   <div class="_toc">
@@ -59,8 +60,8 @@ app.templates.helpPage = """
   <h3 class="_shortcuts-title">Navigation</h3>
   <dl class="_shortcuts-dl">
     <dt class="_shortcuts-dt">
-      <code class="_shortcut-code">#{ctrlKey} + &larr;</code>
+      <code class="_shortcut-code">#{navKey} + &larr;</code>
-      <code class="_shortcut-code">#{ctrlKey} + &rarr;</code>
+      <code class="_shortcut-code">#{navKey} + &rarr;</code>
     <dd class="_shortcuts-dd">Go back/forward
     <dt class="_shortcuts-dt">
       <code class="_shortcut-code">alt + &darr;</code>