add support of numerical field

The new flag  `-n` has been added to provide logs for counterparty. Example output:

```
$ tail -1 messages.log | ~/fixpp.py -d ~/FIX44.xml
[2014-10-22 15:13:46.126] 8=FIX.4.4,BodyLength=215..
```

```
$ tail -1 messages.log | ~/fixpp.py -d ~/FIX44.xml -n
[2014-10-22 15:13:50.198] 8=FIX.4.4,BodyLength(9)=77,MsgType(35)=HEARTBEAT(0)...
```