extend export command to show tombstone + change output format to CSV

merging https://github.com/chrislusf/seaweedfs/pull/610 and add "-limit" option

weed/storage/needle.go

@@ -258,3 +258,7 @@ func ParseNeedleIdCookie(key_hash_string string) (NeedleId, Cookie, error) {
 	}
 	return needleId, cookie, nil
 }
+
+func (n *Needle) LastModifiedString() string {
+	return time.Unix(int64(n.LastModified), 0).Format("2006-01-02T15:04:05")
+}