Extract containers

Well, containers and objects, really. Haven't addressed metadata at all yet. That'll come next once these idioms are solid.

Still a little unclear how I want to represent objects that are created as a result of an API call (and, thus, definitely proxy for resources in the Rackspace cloud) and objects created via the SDK (but are not necessarily representative of objects in the cloud), e.g.,

```ruby
# This is a Container containing data on a server-side container
container = cf.containers.all.first

# versus this Container which is a client-side created container
container2 = cf.containers["foo"]
```

I'm pretty sure I want one consistent idiom for unsynch'd objects (still trying to find a good term for that) vs synch'd ones.
The "override a method to return a Symbol to `#send`" idiom here feels weird to me. I'd find it more readable if it was either:

```ruby
# Methods that you override to return a real object

class PaginatedFiles < PaginatedCollection
  def collection_accessor(collection)
    collection.files
  end

  def marker_from(last)
    last.name
  end
end
```

Or:

```ruby
# Metaprogramming to make them one-liners
class PaginatedFiles < PaginatedCollection
  collection_method :files
  marker_field :name
end
```

The former is more explicit and traceable; the latter is cleaner and more Ruby.