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.
I know. I initially called it `list_containers`. But then that's moving toward having a unique method call on `CloudFiles` for each operation.  I extracted a `Containers` class for organization. And then I wound up with `all` from that.

`list_containers` isn't a particularly idiomatic Ruby method name, though. It just maps to our API.  Instead of using our API names as method names, I'm using `DocsLinkable` to indicate which methods map to which API calls.