Expose Pillow package version as PIL.__version__

[techtonik]

Fixes this:

>>> import PIL
>>> from PIL import ImageGrab
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/PIL/ImageGrab.py", line 22, in <module>
    raise ImportError("ImageGrab is OS X and Windows only")
ImportError: ImageGrab is OS X and Windows only
>>> PIL.version
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'version'
>>> PIL.__version__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute '__version__'
>>> PIL.get_version()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'get_version'
>>> 

[radarhere]

Just to check - there's no reason for importing ImageGrab in the middle, yes?

[techtonik]

@radarhere yes, importing ImageGrab is just demonstration of the problem, which leads to version checking.