[PoC] Mime type guesser

I did a small research about formats problems (discussed here https://github.com/liip/LiipImagineBundle/pull/288#issuecomment-30873520). 

So right now I think that data loader MUST return original image binary. Later in the controller we can wrap it with Imagine Image instance but still we have ability to guess mime type based on the content. The guessed format will be passed to Resolver store method. This way we do not need any image objects at all