Beginnings of Go web server with mongoDb
You might wanted to start writing some comments on your code.

For example I've read all your code but there are some method that I don't understand like : 

func(a* App) GetMongoSession() *mgo.Session {
  return a.Mongo.Session.Copy()
}


Okay we know It is getting the mongo session. but for what purpose? 

// We get the mongo session for .... you can start explain why
func(a* App) GetMongoSession() *mgo.Session {
  return a.Mongo.Session.Copy()
}


I think it is a good think to keep comment the code, so others can understand it quickly. especially when someone who new to mongoDB. :D