Added User model
Added UserResource resource
Explicit assignations and simpler signature
Changed destination path for converted files

The converter won't pollute the directory where files are originally stored any
more. Now it creates a sub directory named after the file extension and stores
the new files there.
Handling the case of filenames starting with a dot
Checking ALLOW_DELETE in a decorator

If the check is in the delete() method, when a resource overwrites the method
the check is lost. This way, we can freely overload any method and the check
will still be performed.
Refactored common logic for fetching items

Each Resource was reimplementing the fetching of items (by id and slug) in a
similar fashion, changing only the model they operated on.
Moved delete around
Bugfix: Headers are a dict
Fixed typo
Removed `methods` attr

Made little sense.
Added POST method to Artist resource