Add amd64 optimized siphash core

This code was produced by gcc from github.com/floodyberry/siphash

benchmark            old ns/op     new ns/op     delta
BenchmarkHash8       25.9          19.9          -23.17%
BenchmarkHash16      33.4          23.8          -28.74%
BenchmarkHash40      56.7          36.8          -35.10%
BenchmarkHash64      80.4          49.7          -38.18%
BenchmarkHash128     140           84.0          -40.00%
BenchmarkHash1K      1003          563           -43.87%
BenchmarkHash8K      7841          4331          -44.76%

benchmark            old MB/s     new MB/s     speedup
BenchmarkHash8       309.45       401.77       1.30x
BenchmarkHash16      479.34       673.41       1.40x
BenchmarkHash40      705.55       1087.64      1.54x
BenchmarkHash64      795.60       1286.88      1.62x
BenchmarkHash128     911.46       1523.03      1.67x
BenchmarkHash1K      1020.03      1817.62      1.78x
BenchmarkHash8K      1044.64      1891.28      1.81x
Merge pull request #4 from dgryski/amd64-core

Add amd64 optimized siphash core
Don't build asm version on appengine.
Build hash.go on appengine.
Add experimental 128-bit siphash output

The changes required for 128-bit output are small, so hash128.go is a
modified copy of hash.go.  I've also included an amd64 assembly core,
similarly modified from the amd64 core for the regular 64-bit siphash.
Patching the assembly core was a bit trickier.

The test cases for siphash-128 are from the reference implementation.

Benchmarks for the pure-Go vs the asm core:

benchmark                old ns/op     new ns/op     delta
BenchmarkHash128_8       35.0          29.0          -17.14%
BenchmarkHash128_16      42.2          33.3          -21.09%
BenchmarkHash128_40      65.5          46.5          -29.01%
BenchmarkHash128_64      88.2          60.0          -31.97%
BenchmarkHash128_128     150           95.1          -36.60%
BenchmarkHash128_1K      1023          576           -43.70%
BenchmarkHash128_8K      7953          4357          -45.22%

benchmark                old MB/s     new MB/s     speedup
BenchmarkHash128_8       228.56       275.44       1.21x
BenchmarkHash128_16      379.05       480.80       1.27x
BenchmarkHash128_40      610.29       860.14       1.41x
BenchmarkHash128_64      725.90       1066.54      1.47x
BenchmarkHash128_128     850.00       1346.35      1.58x
BenchmarkHash128_1K      1000.77      1777.15      1.78x
BenchmarkHash128_8K      1029.93      1879.78      1.83x
Merge pull request #5 from dgryski/hash128

Add experimental 128-bit siphash output
Add 128-bit SipHash to hash.Hash version.
Test on Travic-CI; use Markdown in README.
Tweaks to README.
Remove support for Go 1.0.
Merge pull request #1 from dchest/master

merged latest master from @dchest