Add Binding#getAdapter (so that it could be used in custom binders)

For example:

```js
rivets.binders.toggle = {
  bind: function(el) {
    var adapter = this.getAdapter();

    this.callback = _.bind(function() {
      var value = adapter.read(this.model, this.key.path);
      adapter.publish(this.model, this.key.path, !value);
    }, this);

    $(el).on('click', this.callback);
  },

  unbind: function(el) {
    ...
  },
  routine: function(el, value) {
    ...
  }
};
```

All the changes in `rivets.js` and `rivets.min.js` must be due to newer version of uglify-js (or whichever minifier is being used).