Allow multi-valued references; give tags some love

• New: it is now possible to set references when a migration step results in a list of items, and not just in a single item. The resulting reference will have a value which is an array instead of a scalar value.
  This has to be specifically enabled for each migration step where references are expected to be multi-valued:

    references_type: array
    references:            
        -
            attribute: some_id
            identifier: my_array_ref
  

• New: references are now resolved for in the keyword element for tag/create and tag/update. This makes it easier to mass-create eztag tags in conjunction with the eZLoremIpsum bundle

• New: tags can now be matched by their parent tag id

• New: its is now possible to set references to a tag keyword

• New: it is easier to create/update tags in a single language (the main language of current siteaccess is used if unspecified)

• BC changes:

  • the RepositoryExecutor class now expects subclasses to implement method getReferencesValues and not setReferences any more