Added callback demo for Glacier2

[bernardnormier]

The README.md is missing.

My other comments apply to this Swift version and also other language mappings - may as well get Swift right first!

• We should add more comments, with proper spelling (e.g. Command prompt)
• The callback receiver object ID name should be a UUID, with a comment that the server doesn't need to know about it
• Add a comment for "fake" explaining what it's about (the Glacier2 router routes bi-dir calls to objects in the client only when these objects have the correct Glacier2-issued category)
• we should not create/fill context when doing so does nothing:
  • for twoway requests: _ovrd has an effect only for oneways and batch oneways, while _fwd = t is the default behavior
  • for oneway requests: _fwd is not useful, but _ovrd is
  • for batch oneway requests: _fwd is useful with the default config. I would also just set _fwd explicitly to "O", it's more readable.
• in config.glacier2, we should remove:

  #
  # These two lines instruct Glacier2 to forward contexts both for
  # regular routing, as well as for callbacks (reverse routing).
  #
  Glacier2.Client.ForwardContext=1
  Glacier2.Server.ForwardContext=1

  This demo does not use the forwarded contexts, so it's more confusing than anything to show these properties.

[bernardnormier]

You should also remove the throws from all the operation implementations, since none of them can throw anything.