start work on #45 - set pixel
add setPixel tests
add setPixel method to Image and Batch prototypes
Merge branch 'feature/decree-color-type' into feature/set-pixel
fix typo
add coordinates validation to setPixel
Merge branch 'feature/decree-custom-types' into feature/set-pixel

Conflicts:
	lib/Batch.js
	lib/Image.js
fix setPixel args validation test
Merge branch 'feature/get-pixel' into feature/set-pixel

Conflicts:
	lib/ImagePrototypeInit.js
remove 'setPixel' from random batch test generator

...as it may sometimes fail due to our of bounds coordinates when image
is randomly resized (for example).
adds readme docs and an example for setPixel