IndyDID's restricted to an account

[thiagoromanos]

The current code for IndyDIDRegistry contract, a Indy DID is associated to a account, and an account can have only one DID.
An account must be able to create multiple Indy DIDs, in a case that a single account might have different Indy DIDs for different purposes.

ps.: this behavior only applies to Indy DIDs, because Eth ones are a representation of the account.