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.
Added test
Merge pull request #34 from shuhaowu/chained-with-fields

Added support to chain withfields