Rabbit: Added support for subcollections (rev 2)

[mfojtik]

Hi,

This is the second revision of 'subcollections' patch for rabbit.
In addition to previous patch this:

1. Rename 'subcollection' helper method to just 'collection' so:

collection :buckets do
  collection :blobs do
  end
end

2. Fully rebased against current master

  -- Michal