Wordpress plugin

Update the header comment to follow the standard used by WordPress plugins

* Yes you can use it as a plugin if you really really want to - that's how WordPress distributes packages
* Compatability with [HM Base mu-plugin autoloader script](https://github.com/humanmade/hm-base/blob/master/content/plugins-mu/loader.php)
* Remove contributors - not easy/feasable to keep this up to date. Github is a good record of contributions.
* Why not?