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.
`onDidStopChanging` is a weird name. I have a feeling due to clarity, most people would bind to `onDidChange`. Thinking of a better name...