Added support for verifying distinct messages, signed by distinct entities with an aggregated signature.

[Fronsto]

Resolves #3 ,
Added support for verifying distinct messages. Previously all the messages signed by distinct entities needed to be same.

[asonnino]

We should implement a counter-measure again rogue public key attacks: https://crypto.stanford.edu/~dabo/pubs/papers/BLSmultisig.html

We could go with the simplest, namely ensuring that all messages are different in aggregate_verify.

[Fronsto]

Yeah makes sense. I've updated the code and created a new function sign_unique that appends public key to the message to be signed, and similar modification in aggregate_verify, thus making sure the messages with be different.

[Fronsto]

I've made that change. This will ensure now that the supplied vks in aggregate_verify are different.