Add support for $href

In #1 and #10 we added `$post` and similar accessors on `StoreValue` objects. We could just as easily add an `$href` accessor, which would make using `href` much more readable:

### Current
```Javascript
const authorUri = await this.api.href(book, 'author')
```

### Proposed
```Javascript
const authorUri = await book.$href('author')
```