irssinotifier.pl: only encode to utf8 if terminal is not utf8

Double encoding utf8 messages to utf8 seems to break them. Only
encode if terminal is detected to be non utf8. This also requires
that irssi's recode is on and correctly set up.
Merge pull request #130 from qvr/wtf_encoding

irssinotifier.pl: only encode to utf8 if terminal is not utf8