Updated buster version & added "del" method

Hello, I'm using `loud-accessor` in my project and I needed to have a `del(ete)` method to unset attributes of my models extended with your lib.
I also updated the buster version to run the new tests and fixed the configuration with the new syntax, adding a `spec_helper.js`.

I hope you would like these changes and pull my request in. Thanks for the great work!