The change factors the pure server implementation into the "core"
directory and factors the file driver into "driver/file" and the minio
driver into "driver/minio".
This means that users of this library can import goftp.io/server/core
without having to import the file and minio drivers which may not be
needed.
This also adds a compatibility layer which exports all the types,
functions and variables that were exported at the top level.
This means this change should be 100% backwards compatible.
Fixes#116
