Expose a base constructor for instanceof

For #37. It'd be useful to have a public "base class" to unequivocally identify a modella model:

```js
var model = require('modella')
var Shark = model('Shark')
var s = new Shark()

s instanceof Shark
s instanceof model.Model
```

In this PR the constructor does nothing but lend its name to the proud lineage of all models, but we could move stuff in there if we care about that.