The `density` (or `normed`) parameter was set in the original implementation of `equalize`, but it is unnecessary since `cumulative_distribution` renormalizes values. The only other function in scikits-image that calls `histogram` (`filter.threshold_otsu`) is not affected by renormalization.
`equalize_hist`: "max_intensity" parameter no longer exists---img_as_float normalizes intensity range
`cumulative_distribution`: Return centers of bins instead of the edges.
Move `histogram` function from filter subpackage.
Add test of equalize_hist
Add example of histogram equalization.
Stefan van der Walt