Handle utf-8 completions and r/w encodings for labels and contacts

Updated fix_encoding to try to set the encoding to UTF-8 and check for valid_encoding before doing anything else. The ncurses textfield set the encoding to US-ASCII whatever the input or encoding (valid), by just changing the encoding (without converting) the string becomes valid and subsequent conversions does not try to fix a valid string.

Should fix #117.

Also note for https://github.com/hsanson/sup/tree/gmail_source, include: https://github.com/gauteh/sup/commit/0aa21a0872d1537bd30d449b2fef10b8647d00ff