Add support for creating entites as replicas of other entity

[je-ik]

Example:

 entitites {
   event {
     attributes {
        click: { ... }
        view: { ... }
      }
    }
    eventReplica {
      from: event
    }
  }

This will create two entities with identical attributes.

[je-ik]

Closed via datadrivencz#80