Added possibility to set non-latin1 prettyName.

Okay, after almost a year I just remembered about this issue: https://github.com/Gregwar/ImageBundle/issues/42
I've finally made it work with doctrine's transliterator component, which is in a separate behat's repository now.

@Gregwar, now it's you to decide, isn't it an overhead to add an extra dependency just for utf8-compatible pretty names. As for me, it's a great seo opportunity to set pretty names right from the database, whichever language they written in.
Sorry, forgot. Fixed in the last commit.