Make log buffer logger instance specific.

Was skimming the code and noticed this.

As it is now, if a user created two different buffered logger instances with different output streams, the data would randomly get written to one of the two streams.

Threw together a quick example where all the data comes out of the first stream even though it went into two separate loggers.

    var stream = require('stream');
    var http = require('http');
    var events = require('events');

    var connect = require('connect');

    var s = new stream.PassThrough();
    var log = connect.logger({
        format: 'app log',
        stream: s,
        buffer: 1,
        immediate: true
    });

    var s2 = new stream.PassThrough();
    var log2 = connect.logger({
        format: 'app2 log',
        stream: s2,
        buffer: 1,
        immediate: true
    });

    log({socket: {}}, new events.EventEmitter(), function(){});
    log2({socket: {}}, new events.EventEmitter(), function(){});

    s.on('readable', function(){
        console.log('Stream 1 data:', s.read().toString());
    });
    s2.on('readable', function(){
        console.log('Stream 2 data:', s2.read().toString());
    });