Provides an interface for encoding and decoding
among character sets.

    var Transcoder = require("transcode").Transcoder;
    var Buffer = require("buffer").Buffer;
    var transcoder = new Transcoder(sourceCharset, targetCharset)
    var state = {
        "source": new Buffer(10),
        "target": new Buffer(10)
    };
    transcoder.transcode(state);

The state object can be initialized with and may be updated to include:

 - source: the source buffer
 - target: the target buffer
 - sourceStart: the read head, defaults to 0
 - sourceStop: the read boundary, defaults to start.length
 - targetStart: the write head, defaults to 0
 - targetStop: the write boundary, defaults to target.length
 - error: an error message, if one occurs, one of:
    - "resize"
    - "invalid"
    - "incomplete"
 - message: human readable form of the error
 - nonReversible: an accumulator for non-reversible character
   conversions