See http://sipsimpleclient.com/report/3

MSRP report headers:
 * prepare for how to deal with the session when the reports themselves get lost
 * this seem to be caused by a bug in msrplib, the callback for send_chunk
 * is not properly working when multiple chunks fail to be sent

- allow the FileSelector to compute the hash later, with a specific method
  which will be execute in another thread and which will notify the caller
  using notifications about the progress and when it's done, allowing the
  caller to continue.

- end a session while in accepting state (after 1 dec)
- model file transfer after desktop sharing (use handlers on termination)
  have internal/external handlers (notifications based or file bridges)

- use an ordered dict for keeping accounts in the AccountManager ?!?
  see http://code.activestate.com/recipes/576693/

- NB: make the middleware unicode friendly (ticket #121)
  http://sipsimpleclient.com/ticket/121