Sirupsen/logrus — pull request #34 by shuhaowu (2014-06-09 15:34:17+00:00)
Title: Added support to chain withfields

`WithField` and `WithFields` right now doesn't chain properly. Meaning
that if you do something like:

localLog := logger.WithField("tag", "value")
localLog.WithField("v1", "value").Info("message1")
localLog.Info("message2")

The `v1` will be carried over to `message2`.

With this patch, each WithField/WithFields call are isolated.