Add an option to allow a rule to trigger other rules

User that manage a rule can now allow actions executed by this rule
to recursively trigger other rules.
For example, if you have a rule that automatically publish added content,
and an other rule that sends an email when a content is published,
if autopublish rule is marked as 'cascading', then send mail rule will be trigger

related to pull request https://github.com/plone/plone.contentrules/pull/1