Browse folders with broccoli serve

For debugging, it's incredibly useful to be able to browse the outputted tree as a directory. This PR adds that to broccoli serve - if a URL resolves to a directory, then

1. if it doesn't have a trailing slash, it redirects (current behaviour), then
2. if it has an index.html file, that file is served (current behaviour),
3. otherwise, a folder view is rendered (new behaviour):

![screen shot 2014-05-28 at 1 53 09 pm](https://cloud.githubusercontent.com/assets/1162160/3108642/8bd89e94-e692-11e3-9264-1b5697d70df0.png)

I realise this is quite an opinionated PR but I hope you'll consider merging it, it's helped my productivity a lot. Thanks!