Add option to download depots by providing encrypted gids with password

[Pavel Djundik]

Something like: -app 730 -depot 731 -betapassword test -encryptedmanifest 81C7DA7BC46CFAA3EAB938013C9A5EFD

Alternatively, -manifest param can check for presence of A-F characters in the strings (and by length), since manifest ids are normally numeric.