Tests and access functions for Effects.c

Effects.c has four main functions:

 * ImagingEffectMandelbrot
 * ImagingEffectNoise
 * ImagingEffectPerlinTurbulence
 * ImagingEffectSpread

 
ImagingEffectMandelbrot
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This has Python access functions in _imaging.c, but nothing in Image.py. I added a test and a function in Image.py.

ImagingEffectNoise
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This has Python access functions in _imaging.c, but nothing in Image.py. I added a test and a function in Image.py.

It passes on Windows (and produces a correct-looking image) [but stalls on Travis CI (Linux)](https://travis-ci.org/hugovk/Pillow/builds/34175912). I can't debug it right now, so have skipped the test on non-Windows.

What should be done with this?
 * Debug/fix it?
 * Remove it entirely?
 * Remove the new access function for the time being?
 * Leave it like this?

ImagingEffectPerlinTurbulence
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ImagingEffectPerlinTurbulence is "in progress" since 2003 and does nothing except return NULL. Its helper functions are all behind an #if 0.

I removed it.

ImagingEffectSpread
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This has Python access functions in _imaging.c, but nothing in Image.py. I added a test and a function in Image.py.


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 * Helps towards: Test coverage >= 80% #722.