remove testmode arg and update readme

[Deleted user]

The old testing mode flag was purposed to create and instantly delete repository deposits. Since it is no longer used and necessary, the option should be removed, too.

• remove --testmode option

Also according to this and the latest shipper update, introducing bagit, the readme file must be improved

• remove testmode description from readme
• add bagit requirement and description to readme
• add repo class concept to readme