Add CurentSiteMixin.

This is a mixin for making sure the queryset returned is only for the current Site.  Currently, I'm using it with `ListView`s.  I just did this because I needed it but I'm not sure if this already exists or if there is a better way of doing this.

Anyway, let me know if there are any interests in this one and I'll continue working on this -- I'll figure out how to make it work for content that can be added to multiple sites, and add tests and documentation.  Or, if this already exists somewhere or there's a better way of doing this, please let me know as well so I can use that one instead.