Tee:
  - Constructor now requires a source, processes options, initializes properly
    connected pipelets, allows explicit forks (see below)
  - implement _implicit option to differenciate between implicit and explicit tees
  - set own's source according to _implicit
  - Rename add(), remove(), update(), and clear(), into emit_xxx() for compliance
    with Pipelet interface
  - Create pipelet fork(), though the underlying class is currently Tee but will
    eventually become Fork once the current Fork will be phased-out

Pipelet:
  - fix add()
  - _add_destination(): instanciate Tee with _implicit option

Set:
  - simplify pipelet set()