Add MemoryProxy to library, which allows any raw data to be fed through a Reader to transform it in preparation to load a Model or Collection.
Allow an `ignoreUnknownAttrs` option to be passed to the Model constructor, to ignore any initial data properties that don't match Model attributes (just like the set() method supports).
Change Model's `ignoreUnknownAttrs` config to `ignoreUnknownAttrsOnLoad`.
Add `ignoreUnknownAttrsOnLoad` config to Collection.
Update doc comment in Reader about removing source properties if the `ignoreUnknownAttrsOnLoad` config has been set to false in Model/Collection.