Document/warn against using level and message keys in meta

[aseemk]

Related to #7, I remember this bit us for a while early on, and only now do I realize what happened: if you pass a meta object with level or message keys, those properties get silently overwritten:

https://github.com/indexzero/winston-loggly/blob/v0.6.0/lib/winston-loggly.js#L103-L107

  var self = this,
      message = winston.clone(meta || {});

  message.level = level;
  message.message = msg;

Reserving those keys is cool. Just consider documenting this, but better yet, consider logging a warning (maybe just once) if the meta object has those keys. =)

Thanks!