Media.Piece: really use uuids for ids, remove old code.

Remove the old code that used moment() and move uuid to defaults.

Signed-off-by: Adrian Pardini <github@tangopardo.com.ar>
Merge pull request #131 from inaes-tic/crear/uuid-ids-for-real-2

Media.Piece: really use uuids for ids, remove old code.

Previous discussion was at #126