Config to change web directory

Hi,

first of all thanks for this bundle; it's very useful and maybe the best for image manipulation with auto-updated cache.

I had to change my Symfony directories hierarchy 'cause of a shared hosting. According to http://symfony.com/doc/current/cookbook/configuration/override_dir_structure.html , I changed my structure but ImageBundle stop working.

I think this commits can align ImageBundle to Symfony configuration.