_byCid and inheritance

[sarink]

It appears this repo has gone dormant. I've been using this plugin for awhile, and fixed a couple bugs.

1. Since _byCid was deprecated, the _updateModelMembership function broke. This has been fixed.

2. When extending from a parent collection, the subset was not inheriting any extra properties that may have been set on it. This has also bee fixed.

Fork is here: https://github.com/sarink/Backbone.Subset - I will try to keep updating Pau's code as necessary because I still like this little subset library!