Added GrayscaleFilter.

The server now applies another filter - the Grayscale filter. The client
can now see the two applied filters.
Also fixed a bug with the ChechSession HTTP request of the
ImageController - unneeded space removed.
Added a Red, Green, Blue and Combo Gamma filters.
Added a Red, Green and Blue Color filters.
Added Sepia Filter.
Added a custom Contrast Filter with two effects

If the Contrast Filter is applied with true then it also has a little
bit of a blur effect.
If the Contrast Filter is applied with false then the contrast of the
image is fixed and there is no blur, but a smooth result image.
Also rearanged the filters' order so that the more beautiful one are on
the top.
Added two Brightness filters.

Added a positive and a negative Brightness Filters.
Added click2download functionality.

Now when the user clicks on the filtered image he can download it in its
full size.
Refactoreed Download Controller.
Added db.