addSlash = function( str ) {
		return str + ( ( str.length > 0 && str.charAt( str.length - 1 ) === '/' ) ? '' : '/' );
	}
Construct a url for the collection, or for a set of models in the collection if the `models` param is used.
Will attempt to use its own `urlRoot` first; if that doesn't yield a result, attempts to use the `urlRoot`
on models in the collection.

@param {Backbone.Model[]|string[]} [models]