Improve performance of MultiplyElementwise
Improve performance of MultiplyElementwise

+ Add special uint8 treatment to MultiplyElementwise,
  about 1.7x speedup (compared to dtype-tolerant augmenter)
+ Add special treament of boolean mask multipliers in
  MultiplyElementwise,
  about 2.5x speedup (compared to dtype-tolerant augmenter)

aleju/ImageAugmenter,aleju/imgaug,aleju/imgaug