Handle out-of-charset unicode chars (See #7).

mpd uses proper UTF-8 encoding for all its fields;
on the other hand, lcdproc only uses "1 byte charsets": it expects
all incoming messages to be encoded on 8bits exactly.

Most LCDs only support 256 different characters, reusing the control
part of ascii and iso-8859-* charsets.
lcdproc supports this by mapping characters from various standard
charsets to the related LCD-specific codepoints.

All this means that mpdlcd *must* send characters in the proper encoding
expected by lcdproc's charmaps.

If some text (song title, artist name, ...) doesn't appear in the
current lcdproc input charset, mpdlcd will try to convert it (e.g
transform 'é' into 'e', etc.).

This might not work properly for some characters, mostly
korean/chinese/japanese ones.