I have seen some link here.but I have not found anything interesting
I'm not quite satisfied with those explanations. When we create a word with "er" to mean a person who does something, the verb usually comes first. For instance, "bake", I'm sure came before "baker", because the act of baking had to be invented before the word "baker" can be born. The same is true for "hitter", "driver", "swimmer", runner", "programmer", "painter", and so on... The verb has to come first.check this

2. http://www.youtube.com/watch?v=WtNusl2CKM0