more changes related to advertisements.
added position to advertisements. with it, one can choose to either show an horizontal add only at top, bottom or both.
removed can_see_ads check from AdvertisementsHelper::print_advertisement() because it's already checked before calling it.
horizontal-bottom ads in post#index are centered.
removed "Reset hit count" checkbox in ads#edit.
Width and Height in ads form are now "number" fields.
Width and Height must now be set also for "Html" ads.
made ads#show a little nicer.
ads#create and ads#update redirect to ads#index if success.
some other related changes.

made ad#show a little nicer