Different prefix bytes for testnet. Fixes #2.

Now testnet hashes are prefixed with correct bytes:

```
vagrant@millstone:~/github/pycoin$ genwallet -wf ~/bip0032/mk | head -c 2 && echo
cT
vagrant@millstone:~/github/pycoin$ genwallet -af ~/bip0032/mk | head -c 2 && echo
n4
```

Output of ``./test-py2.7.sh``: http://nn.lv/50dm
Output of ``./test-py2.7.sh`` of current mainline master: http://nn.lv/cf85
Delta: http://nn.lv/cf85