Old & copied SHA-3 code and misleading reference to SHA-3 instead of Keccak-256

[jimenezrick]

This blog explains the issue: https://medium.com/@ConsenSys/are-you-really-using-sha-3-or-old-code-c5df31ad2b0. Ethereum uses Keccak-256, not SHA-3, which was standardized afterwards.

I could myself prepare a PR with this improvements:

• Add to Godeps golang.org/x/crypto/sha3 with a small patch to expose Keccak-256: https://github.com/jimenezrick/crypto/commit/8802e1c340c41c8d290e6315609cfbcbdf1b6229 (this thin fork could be hosted in the Ethereum Github obviously)
• Using upstream crypto/sha3 gives some extra performance, tested running go test benchmark.
• Remove old copied SHA-3 code.
• Rename any code referencing SHA-3 instead of Keccak-256

[karalabe]

Fixed per your PR.