Added Nunjucks adapter

I've added support for [Nunjucks](https://github.com/jlongster/nunjucks), a dynamic template engine with a lot of goodies you've come to expect from template engines (loops, includes, blocks, etc.).  

Unfortunately, the ["should render with client side helpers" test](https://github.com/carrot/accord/blob/nunjucks/test/test.coffee#L73) isn't passing yet.  There seems to be an [issue](https://github.com/jlongster/nunjucks/issues/129) with the client-side JavaScript.

I'll also be writing thorough documentation tomorrow.  Let me know what y'all think :+1: