pywrap: Add wrapping around Convolution kernel

Now the python bindings converts python two dimensional array into
GP_FilterKernel2D structure, yay!

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Add Convolution examples to image generator.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Update/Add convolution docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>