Reworked on how storage are determinde

1. Move storage into backends/storage
2. Renamed the function in backend.__init__ to get_storage_backend and get_metadata_backend
3. Reworked on how to initialize storage on field
Move metadata.get_backend to field instead of file
Changed ValueError message
Cleanup newline and default value for METADATA and STORAGE
Merge pull request #33 from SeiryuZ/storage-backend

Reworked on how storage are determinde