update packet endoding to ZRE 2

Seems to be correct, but untested.

I tried to test this, but something is broken beyond my python understanding (did I mention that I hate python?):

```
$ python3
Python 3.4.2 (default, Oct  8 2014, 13:08:17) 
[GCC 4.9.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pyre
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/src/zmq/pyre/__init__.py", line 3, in <module>
    from .pyre import Pyre
  File "/usr/src/zmq/pyre/pyre.py", line 11, in <module>
    from .zsocket import ZSocket
ImportError: No module named 'pyre.zsocket'
```