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.

entry.go

diff --git a/entry.go b/entry.go
index 8cd312e..052697c 100644
--- a/entry.go
+++ b/entry.go
@@ -38,15 +38,23 @@
 }
 
 func (entry *Entry) WithField(key string, value interface{}) *Entry {
-	entry.Data[key] = value
-	return entry
+	data := Fields{}
+	for k, v := range entry.Data {
+		data[k] = v
+	}
+	data[key] = value
+	return &Entry{Logger: entry.Logger, Data: data}
 }
 
 func (entry *Entry) WithFields(fields Fields) *Entry {
-	for key, value := range fields {
-		entry.WithField(key, value)
+	data := Fields{}
+	for k, v := range entry.Data {
+		data[k] = v
 	}
-	return entry
+	for k, v := range fields {
+		data[k] = v
+	}
+	return &Entry{Logger: entry.Logger, Data: data}
 }
 
 func (entry *Entry) log(level string, levelInt Level, msg string) string {