NPM module

This better exposes Buckets as an NPM module, so one can do:

```
npm install buckets
```

and

```
buckets = require('buckets');
buckets(config);
```

This will help in:

 - Moving Grunt/Browserify plugins to devDependencies (and using `prepublish` hook to build before doing a release), so `npm install --production` is much lighter, and general users don't need to compile anything
 - Allowing Buckets to be used as middleware for an existing Express app