Support for verifying a signature

[jnaviask]

It would be very useful if we had a function like verifySignature(message, signature, publicKey) that allowed us to check that signature was the result of signWithPrivateKey(message, privateKey) for the private key corresponding to publicKey.

This would allow the possibility of secure offline signing, especially using gaiacli once this is resolved: cosmos/cosmos-sdk#4581. Thanks!