Update series.updated_at/by fields when image is added

[php-coder]

When we're adding new image to a series we should update it's updated_at and updated_by fields. Also this could be used in a report from #121 -- admin could see what series were updated.

(Removal of the image will be implemented as part of #282)

[php-coder]

@AleksSPb I hope that you woudn't mind if I reassign this issue to @cssru