setup.py: make libbe._version optional.

setup.py: make libbe._version optional.

Thanks to Timmie for pointing this out:

From: Timmie <timmichelsen@gmx-topmail.de>
Date: Thu, 25 Oct 2012 09:42:23 +0000 (UTC)
Subject: Re: [Be-devel] NotImplementedError with Bazaar

...
I notices the following glichtes during the install:

D:\be-be-master\be-be>python setup.py install
WARNING:root:unable to import libbe._version: No module named _version
Traceback (most recent call last):
  File "setup.py", line 6, in <module>
    from libbe import _version, version
ImportError: cannot import name _version
...