add a new 'overview' MVC example

Former-commit-id: f73cbf6010595c639f6c5b5119e2ec41bc9802a5

_examples/mvc/overview/database/database.go

@@ -0,0 +1,22 @@
+package database
+
+import (
+	"app/environment"
+)
+
+// DB example database interface.
+type DB interface {
+	Exec(q string) error
+}
+
+// NewDB returns a database based on "env".
+func NewDB(env environment.Env) DB {
+	switch env {
+	case environment.PROD:
+		return &mysql{}
+	case environment.DEV:
+		return &sqlite{}
+	default:
+		panic("unknown environment")
+	}
+}