Add & rename a few assembly instructions
Pave the way for vertical sse2 scalers
Add sse2 vertical scaler for 2-taps

benchmark                   old ns/op    new ns/op    delta
BenchmarkVerticalScaler2        54203         5320  -90.19%
Add sse2 vertical scaler for 4-taps

benchmark                   old ns/op    new ns/op    delta
BenchmarkVerticalScaler4        93305         7970  -91.46%
Add sse2 vertical scaler for 6, 8, 10 and 12-taps

benchmark                    old ns/op    new ns/op    delta
BenchmarkVerticalScaler6        131407        10500  -92.01%
BenchmarkVerticalScaler8        172609        11735  -93.20%
BenchmarkVerticalScaler10       200411        14370  -92.83%
BenchmarkVerticalScaler12       238813        16490  -93.10%
Add sse2 vertical scaler for n-taps

benchmark                   old ns/op    new ns/op    delta
BenchmarkVerticalScalerN       341219        19451  -94.30%
Mention SS2 optimisations in the documentation

Performance is up ~8-10x

benchmark                        old MB/s     new MB/s  speedup
BenchmarkImageBilinearUp-4         758.22      6770.18    8.93x
BenchmarkImageBicubicUp-4          485.37      5251.98   10.82x
BenchmarkImageLanczosUp-4          368.29      3338.94    9.07x
BenchmarkImageBilinearDown-4       123.20      1433.65   11.64x
BenchmarkImageBicubicDown-4         74.68       791.44   10.60x
BenchmarkImageLanczosDown-4         43.84       538.60   12.29x
BenchmarkImageBicubicIUp-4         478.94      5063.86   10.57x
BenchmarkImageBicubicIDown-4        79.69       769.75    9.66x
BenchmarkImageBicubicRgb-4         240.74      2473.29   10.27x