Check stream types

... so that you cannot
```javascript
var src = getMyTileSource();
var wut = tilelive.createReadStream(src, { type: 'put' });
```

Worth keeping in mind that duplex and transform streams will register as `instanceof stream.Readable` but will not register as `instanceof stream.Writable`. [From the docs:](http://nodejs.org/api/stream.html#stream_class_stream_duplex_1)

> Since JavaScript doesn't have multiple prototypal inheritance, this class prototypally inherits from Readable, and then parasitically from Writable.