Add __version__ to python wrapper

[bfulkers-i]

No description provided.

[ev-mp]

This was a long-thought attribute indeed.
Thank you for the contribution!

[zbynekwinkler]

According to https://github.com/IntelRealSense/librealsense/wiki/Release-Notes this PR is part of 2.31 release. However, I do not see the attribute:

$ pip list
Package       Version    
------------- -----------
numpy         1.17.4     
opencv-python 4.1.2.30   
pip           9.0.1      
pkg-resources 0.0.0      
pyrealsense2  2.31.0.1235
setuptools    39.0.1     

$ python
Python 3.6.9 (default, Nov  7 2019, 10:44:02) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pyrealsense2 as rs
>>> rs.__version__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'pyrealsense2' has no attribute '__version__'