WIP comments

@tenderlove as discussed [here](https://groups.google.com/d/msg/rubyonrails-core/-/ynWCfU9FuVgJ)
my first draft of comments
Still has evil string manipulations but is the interface/direction correct ?

```
scope = User.where(aaa).comment("XXX")
scope.comment("YYY")
scope.to_sql == "SELECT * FROM users WHERE xxx /* XXX YYY */"
```