Expose base constructor for ES6

Should be able to extend errors in a cool way.

``` js
import error from 'lvivier/error'

// extending
class BadRequestError extends error.Error {
  constructor (msg) {
    super(msg)
    this.statusCode = 400
  }
}

// factory-ing
let BadRequestError = error('BadRequestError', {statusCode:400})
```
