Formatting
Add each_value to HashContainable
Add comment in PostagDiff about its current status
Refactor WordDiff diff handling
Distinct between GenericDiff and PostagDiff
Add HashContainable#take

Returns an array of container elements, identified by their id. (like
Hash#values_at)
Add spec for Diff#diff
Minor changes in GenericDiff to satisfy the specs
Compute detailed postag differences
Expose the container as diff ...

... in SentenceDiff and WordDiff