Added histogram equalization

Signed-off-by: Luiz Carlos Cavalcanti <cavalcanti.luiz@gmail.com>
Separated grayscale filter on a separate class

Signed-off-by: Luiz Carlos Cavalcanti <cavalcanti.luiz@gmail.com>
Added standalone capabilities to BoxBlur filter

Signed-off-by: Luiz Carlos Cavalcanti <cavalcanti.luiz@gmail.com>
Added standalone capabilities to Gamma filter

Signed-off-by: Luiz Carlos Cavalcanti <cavalcanti.luiz@gmail.com>