Fixes #17 : Add a ChangeInterface
Merge pull request #18 from Taluu/change-interface

Fixes #17 : Add a ChangeInterface
Check if the value is not already an instance of AbstractSnapshot
Add a CollectionSnapshot
Add PropertyAccess component in the CollectionSnapshot

[reset_minion]
Add an option to the CollectionSnapshot for snapshotting subelements
Add some tests for the CollectionSnapshot
Update the docs

[reset_minion]
Merge pull request #22 from Taluu/collection-snapshot

Collection snapshot