Clean all the things

This release is mainly just tidying up things based on linting and opinions. So I have removed silly things such as that almost blank .jshintrc file which stopped your own ~/.jshintrc working when you were editing any EventEmitter code.

@desandro also submitted a pull request about a bug he found when nesting listeners. I have since improved the tests hugely but still can't reproduce the issue he found. I have merged in that change anyway because it should make the code more robust.

So, no new features, just hardening and cleaning. Enjoy!

Once and scoping

This release has added support for the addOnceListener method and it's alias, once. You can also now set the return value that is checked for when auto removing listeners with setOnceReturnValue.

There are also a few scoping improvements so listeners will now be executed in the context of the EventEmitter instance, not null.