* Add ComputeDomainVerifySigningRoot
* Update validate aggregate proof signatures to use it
* Update verify proposer slashing to use it
* Update test to use it
* Merge branch 'master' into domain-and-signingroot
* Quick fix on test
* Merge branch 'domain-and-signingroot' of github.com:prysmaticlabs/prysm into domain-and-signingroot
* Merge branch 'master' of github.com:prysmaticlabs/prysm into domain-and-signingroot
* Merge refs/heads/master into domain-and-signingroot
* Merge refs/heads/master into domain-and-signingroot