Bit.ac, which is mentioned above, isn't a good wallet, there are many complaints about this service.
You're right, cold storage is a safest place to keep coins. Hardware wallet can be a good option, but currently there are no wallet which would support all your mentioned coins.
Another safe place to keep coins - download official wallet of coin, where you will have full control of coins.
There are also some multi-coins wallets, like Exodus or Coinomi, but again, they only support part of your mentioned coins.
Was looking at exodus. Couldn't they steal them too since it's a web based app?