Mirrored observable collections

[Mike Hearn]

Someone got in touch with me today and pointed out a bug in this code, but
it was already fixed by me some time ago. I've refreshed the gist with the
latest versions of these classes, but the upstream project is now fully
open source. You can get the latest code here:

https://github.com/vinumeris/lighthouse/tree/master/common/src/main/java/lighthouse/threading

I should probably turn this into a little self contained library at some
point.