Removed default value 'fineuploader' for frontend key in Configuration.

This was a legacy decision which will be changed in 1.0.0.
The first version of this Bundle only supported Fineuploader which
would have been a BC break if 'fineuploader' was not the default value.
Removed a notice which was stating that there is a default value for fronend config.
Made request passing in ValidationEvent mandatory.
Fixed "unused code" labels in Scrutinizer report.
Added Next Steps section to frontend documentations.
gaufrette chunk storage
separated storages both in definition and configuration
moved the metadatasupporter check
better file size calculation for gaufrette files
fix non-chunked uploads and improve extension handling
fixed messy type hinting for more comprehensible structure
made scrutiziner happier
broke configuration up even more fro scrutinizer
had to lower interface requirements for the controllers
fixed existing tests and some mistakes revealed by them
don't test for sf2.1
Update configuration_reference.md
Update chunked_uploads.md
Update gaufrette_storage.md
Update chunked_uploads.md