Multi-site support

This pull request adds support for serving multiple sites (with different URLs) from the same Refinery instance. It is still a bit rough around the edges in terms of UI support (or lack thereof), but it is being used on some of our production sites and is working well.

I mainly want to see if there is any interest in getting something like this into core Refinery. If there is, I would love to work some more on polishing this up and getting it merged.