Rename RealVirtualFileSystem to NativeFileSystem
First implementation of RepositoryFileSystem

The net.secloud.core.RepositoryFileSystem implements the
net.secloud.core.VirtualFileSystem trait and allows easy accessing of
data in a commit.

Remove VirtualFileSystem methods openRead and openWrite.
Add NullFileSystem

Simple stub for a filesystem, that contains no elements.
Make VirtualFile independent from VirtualFileSystem