pipelet.js: ..
  - implement Event_Emitter class extracated from Pipelet
  - Pipelet derives from Event_Emitter
  - Transaction derives from Event_Emitter
  - Transaction..end() emits event 'ended'
  - Transactions..get_transaction() listens to Transaction event 'ended' to remove from transactions
  - Transaction..end() sets this.ending to true if it cannot end immediatly
  - Transaction..emit_add() and ..emit_remove() calls ..end() if this.ending
  - Transaction.._get_name() retrieves transaction's pipelet name
  - Transaction..set_source_options() allows to set source options when reusing transaction from upstream pipelet
    in order to capture and set this.source_more
  - Transaction..get_transaction(): call this.set_source_options() when reusing upstream or nested transaction
  - Comment-out deprecated XS.more() and XS.no_more() and associated tests
  - Add more Transaction tests, total automated tests now 190