text: Title: No `tensorboardX.__version__` attribute
  Body: Title says it all... it would be nice to have a `__version__` attribute on this module. I'm running under Python 3.6.6.
  
  ```>>> import tensorboardX
  >>> tensorboardX
  <module 'tensorboardX' from '/home/dave/.conda/envs/dk_env/lib/python3.6/site-packages/tensorboardX/__init__.py'>
  >>> tensorboardX.__version__
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  AttributeError: module 'tensorboardX' has no attribute '__version__'```