Remove silent option from Model#set.

The most common use case for the `silent` option is to prevent specific handlers from being called for performance or UI reasons.  The problem with this is that *all* handlers are silenced, not just the one in question.  This can always be better handled with custom options, which could even be named `silent`.  ;)

```js
var model = new Backbone.Model;
model.on('change', function(model, options) {
  if (options.silent) return;
});
model.set(..., {silent: true});
```

In all cases I'm aware of, silencing change notifications is an anti-pattern that should not be supported.

@tgriesser I would love a second pair of eyes on the tests to make sure I didn't remove any legitimate cases.