Realms appear to be working. So far, realms are a way of talking about
related clusters. They prepend their name to the name of the clusters
within them and also allow some things (environments, common cloud
properties) to be merged into each cluster.

For example,

Ironfan.realm(:p1) do
  cloud(:ec2).bits 64

  cluster(:control) do
     # etc.
  end
  cluster(:kfk) do
     # etc.
  end
end

the realm here would result in p1_control and p1_kfk clusters with a
default cloud(:ec2).bits attribute merged in.
tidying up a bit
Merge branch 'jbro_realms' of https://github.com/infochimps-labs/ironfan
Version bump to 4.12.0
Regenerate gemspec for version 4.12.0