Fixed a regression in AWS::CloudFormation::StackCollection#each.

When addressing GitHub issue #419, a regression was introduced that
prevented memoization of `AWS::CloudFormation::Stack` attributes
when retrieved from `StackCollection#each`.

The regression was caused by switching from using the `#describe_stacks`
api operation to the `#list_stacks` operation to enable filtering.  This
also had the side effect of returning all (including deleted) stacks
in the default enumeration.

The `#each` method now will only use the `#list_stacks` api operation
when a filter is present.  Additionally, memoization logic has been
added to `Stack` to enable caching attributes returned by
the `#list_stacks` api operation in addition to those returned
by `#describe_stacks`.

Added an integration test to ensure this does not regress.

Fixes #423
Fixes #424