Merge pull request #3 from MidnightWonderer/feature/reduced-round

modify to keep the old constructor of full rounds ChaCha20

Add rounds constraint

implement all available test vectors

add tweakable rounds variable to the cipher

Rename Nonce24 -> XNonce.

Merge pull request #2 from Yawning/xchacha20_squashed

Add support for XChaCha20, the extended nonce variant.

Merge pull request #1 from Yawning/intel_perf

Double performance on modern Intel processors.

Test on Go 1.4rc1.

Only test on Go 1.3.3.

Test on Go 1.3+.

Re-organize chacha20.go.

Rename NewCipher to New.