Add primary keys to example application
Update ActiveResource::Base to work with multiple belongs-to associations and improve default lookup of providers.
Add first and last properties to ActiveResource::Collection, which return the first and last members of a collection, respectively.
Add spec for complexly associated models, and add spec for syntactic options when writing models.
Add example of provider name lookup, where the provider has the name of the property it's intended to represent on the model (e.g.:

	function Post(data) {
		this.hasMany('comments');
	}

This syntax is all we need if the comments provider is named 'Comment', and not something else.