Remove reference to Bento hookfile.
Added a function for Gamma Correction
Prevented info loss, implemented Log and Sigmoid Correction
Separating the functions
Separating correction methods into different functions
Correcting code quality issues
Added better tests and removed weak ones
Added stronger tests
Corrections and Improvements
Minor improvements in docs, tests
Error handling for images with negative values in Image Correction methods
Adding a utility function to test non-negativity of an image
Renaming rescale_intensity_* as adjust_*
Replacing every occurence of substract* with subtract*
Merge pull request #581 from ankit-maverick/subtract

STY: Replacing every occurence of ``substract*`` with ``subtract*``.
Final stylistic changes in exposure.py
Radon transform example: Correct axis labels
Merge pull request #570 from stefanv/bento_hookfile

BLD: Remove reference to Bento HookFile.
Radon transform example: Remove semicolons.
Merge pull request #583 from josteinbf/radon-example-axes-fix

Radon transform example: Correct axis labels