Debian 7 (wheezy) support for btrfs RAID

I've been able to run the Debian 7 installer (beta1) and get a working Debian system on btrfs RAID1 root FS.

A few manual steps and patches required - it would be useful to get feedback about this process. I might have a go at patching partman to fully support this through the installer menu.

I've written up the process as an install report bug:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=686130

Any feedback is welcome.