Added joining & leaving handling.

* Program.cs:
  Added OnJoin and OnLeave handlers.
  Removed limited message count handling.

* IRCClient.cs:
  Documentation updates.
  Updated JoinDel and LeaveDel delegates.
  Added separate channel catcher, so parsing JOIN's and PART's are
  possible.
  Added JOIN and PART message handlers.

* Extensions.cs:
  Added overload Range() method that can take multiple ending strings.