Banner: test and preview select image using html 5 file api.

Up to now, the for is only telling the user whether the image
has the right size or not. Also, it only shows the image input
field after the user has selected what kind of banner they want
to upload an image for. Using ajax to get the image's dimensions
for the kind of banner the user selects.
Save banners. Saves img on disk, name on db, etc...

Used html5 file api. Still have to validate some
things with javascript.
List banners. No databales of any other script.
Change table banner_type code that depends on it.

banner_type renamed to banner_types to match the style
of the application since all other tables are in the
plural form.
Update banner details, with or without a new image.

Javascript sets a value in a hidden field that tells the
model whether the user has choosen a new image for the banner
or not. If so, removes the old image from the HD, and updates
its new rand_name in the database, otherwise, doesn't touch
the banner itslef and just update the other fields.