View-specific Adapters

View-specific adapters. Pretty simple. Libraries can provide their own adapters:

```js
var Observable = require('observable');
var view = reactive(el, model);
view.adapter = Observable.adapter;
```

Same interface for global reactive adapter so this still works

```js
reactive.adapter = Observable.adapter;
```

To make it affect all Reactive instances. If we get that `use` pull request in, it will be cleaner:

```js
reactive.use(observable.adapter);
```