[1.0] Rework data loaders. Introduce 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 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

__UPDATE__

Changes done in this PR:

* Introduced `MimeTypeGuesserInterface` and its simple implementation `SimpleMimeTypeGuesser`. 
* LoaderInterface:find returns string - binary content of the image (In future we can allow to return RawImage instance)).
* DataManager:find always returns `RawImage` instance. The plan is to reuse this RawImage in other places like filter manager and cache resolver.
* Remove dependency on imagine service in all loaders.