Don't register the filesystem as a service
Renamed the upgrade-file

According to the symfony-standards the file should be named as "UPGRADE_<version to upgrade to>".
Use native PHP methods for file/dir existence.

Instead of using the filesystem-component for basic filesystem-checks use the
native ones: file_exists, is_file, is_dir.

Before:
	```
	SpoonFile::exists(...)
	```

After:
	```
	// depending on the situation you should use
	is_file(...)
	// or
	is_dir(...)
	// or
	file_exists(...)
	// or create a filesystem-object
	use Symfony\Component\Filesystem\Filesystem;
    use Symfony\Component\Filesystem\Exception\IOException;
    ...
	$fs = new Filesystem();
	$fs->exists(...)
	```

In most cases I chose is_file instead of file_exists because is_file will also
check if a path is a real file.
mkdir
IOException in favor of SpoonFileException, also a bit of cleanup
SpoonFile::exists & SpoonDirectory::exists have the same replacement