Add support for an encoding fallback.

Added a FallbackEncoding property to IrcConnection. If IrcConnection's bytes-to-string decoding fails using the encoding stored in Encoding, decoding using FallbackEncoding is attempted. (The FallbackEncoding will, very probably, fail silently.)

The default FallbackEncoding is Windows-1252, a strict superset of ISO-8859-1 and a rather conservative choice.