Add support for UTF-8-with-fallback decoding.

Added an EnableUTF8Recode property to IrcConnection. If set to true, everything
is sent using UTF-8, and IrcConnection's bytes-to-string decoding attempts to
use UTF-8 first, and the encoding specified in Encoding next. (If false,
messages are sent and received using Encoding.)
Merge pull request #8 from RavuAlHemio/experiments/fallback_encoding