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

web/rest.go

@@ -5,7 +5,9 @@ import (
 	"net/http"
 )
 
-func RenderJson(w http.ResponseWriter, data interface{}) error {
+// RenderJSON sets the correct HTTP headers for JSON, then writes the specified
+// data (typically a struct) encoded in JSON
+func RenderJSON(w http.ResponseWriter, data interface{}) error {
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 	w.Header().Set("Expires", "-1")
 	enc := json.NewEncoder(w)