Add event subscription methods

The `::on` method will cause deprecation warnings only if `TextBuffer.deprecateOn` is set to true in Atom. This is a precaution in case we want to upgrade `TextBuffer` on master before atom/atom#3418 lands.
I think we should either order this above `onDidChange` or put some text in both `onDidChange` and `onDidStopChanging` doc strings indicating that `onDidStopChanging` is almost always what the user wants.