Add a "replace" option to Backbone.Model.set

I needed to update some of a Model's data that came in from the server. Currently, there are two options: call `Model.set`, or set `Model.attributes` directly. I didn't want changes to be recorded, but I also couldn't use `silent` because I needed the respective views to update. So, I wrote a monkey patch:

``` coffeescript
Backbone.Model.prototype.reset = (attributes, options) ->
attrs = attributes || {};
if options.parse
attrs = this.parse(attrs, options) || {}

@set(attrs, options);
@changed = {};
```

Wondered why `Backbone.Model` doesn't have a reset method like `Backbone.Collection`?

Start with Backbone.Model.set and compare the requested behavior with Backbone.Collection.reset. Read the 22-comment discussion to determine whether the API should be a replace option or a Model.reset method, then define the behavior and add coverage showing that views update without recording changes.

Written by the indexing model from the issue text.