Comments
Remove now useless custom validation
Rename the pictures private method to picture_sizes
Use picture_sizes everywhere
Read slide picture sizes from current event
Refactor the generate_images

Previously we had all kinds of entrypoints to alter the image generation,
now we rely on more semantic generate_full_image and generate_previews private functions.
preview image size had its height and width mixed up

We were generating images 225px wide instead of 400px
More robust logic on the slide image linking

Previously we relied entirely on to_full local variable in the view, now we
also check if this is slides#show action and then render link to full slide image.

This is done because render @slide doesn't process the locals: definition at all.
Remove unprocessed locals: definition

When using render @object shorthand rails doesn't process the locals: option hash at all.
Resize the previews to the exact size configured

Previously we were resizing keeping aspect ratio, now we rely on user configuring the sizes properly.
Remove rendering of slide.position

Since ranked model uses huge values for positions rendering them to users makes no sense.
Add class_name: option for RankedModel

With sti-classes the ranked model automatically scopes with type also and we don't want that
Add a way to determinate slide position inside mastergroup

RankedModel needs some sql-magic to count the order of a slide inside a group.
We implement it in group_position function
Use delegate instead of custom wrapper function

We previously had a custom helper for group.name, now we delegate it to master_group
Use has_many :through associations

We previously had helper methods instead of proper associations, now we use less glue and more framework features instead.
Add presentation_position method

This allows us to deal with RankedModels sparse indexes and return the real position of the group inside a presentation
Ruby 1.9 style hashes for readability
Use has_many :through instead of custom methods

We can do the .presentations associations the rails way instead of custom code, so do it so