Associate media files with deck

[gflohr]

At the moment, you can only do one import per colour because the media files are associated with one colour (black or white). It would be better to also encode the deck id into the name so that we can allow a structure of decks.

[gflohr]

It is not necessary to encode the colour into the name because each deck is associated with a colour. It is sufficient to encode the deck id into the filename.

[gflohr]

Fixed with PR #14.