Follow meta-linter recommendations for all of Inbucket

- Rename BUILD_DATE to BUILDDATE in goxc
- Update travis config
- Follow linter recommendations for inbucket.go
- Follow linter recommendations for config package
- Follow linter recommendations for log package
- Follow linter recommendations for pop3d package
- Follow linter recommendations for smtpd package
- Follow linter recommendations for web package
- Fix Id -> ID in templates
- Add shebang to REST demo scripts
- Add or refine many comments

themes/integral/templates/mailbox/_list.html

@@ -1,6 +1,6 @@
 {{$name := .name}}
 {{range .messages}}
-<div class="box listEntry" id="{{.Id}}">
+<div class="box listEntry" id="{{.ID}}">
 	<div class="subject">{{.Subject}}</div>
 	<div class="from">{{.From}}</div>
 	<div class="date">{{friendlyTime .Date}}</div>

themes/integral/templates/mailbox/_show.html

@@ -1,11 +1,11 @@
 {{$name := .name}}
-{{$id := .message.Id}}
+{{$id := .message.ID}}
 <div id="emailActions">
   <a href="/link/{{$name}}/{{$id}}">Link</a>
-  <a href="javascript:deleteMessage('{{.message.Id}}');">Delete</a>
-  <a href="javascript:messageSource('{{.message.Id}}');">Source</a>
+  <a href="javascript:deleteMessage('{{.message.ID}}');">Delete</a>
+  <a href="javascript:messageSource('{{.message.ID}}');">Source</a>
   {{if .htmlAvailable}}
-    <a href="javascript:htmlView('{{.message.Id}}');">HTML</a>
+    <a href="javascript:htmlView('{{.message.ID}}');">HTML</a>
   {{end}}
 </div>
 <table id="emailHeader">