Package missing __version__ attribute

[piskvorky]

It seems we're missing the standard __version__ attribute, so users could check their installed package version:

In [1]: import smart_open; print(smart_open.__version__)
No handlers could be found for logger "smart_open.ssh"
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-1-e26c8ec8e852> in <module>()
----> 1 import smart_open; print(smart_open.__version__)

AttributeError: 'module' object has no attribute '__version__'

(ignore the "No handlers found…", that's a separate issue #288 ).

We should definitely include __version__, same way we do for Gensim etc.