Detect SSE automatically and enable corresponding optimized code path.

With this change, following happens.

```
if sse {
   compressSSE()
   return
}
compressGeneric()
```

compressGeneric is used as a fallback when SSE is not detected.

cpuid.go

@@ -0,0 +1,29 @@
+// +build 386,!gccgo amd64,!gccgo
+
+// Copyright 2016 Frank Wessels <fwessels@xs4all.nl>
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+package blake2b
+
+func cpuid(op uint32) (eax, ebx, ecx, edx uint32)
+
+// True when SIMD instructions are available.
+var sse = haveSSE()
+
+// haveSSE returns true if we have streaming SIMD instructions.
+func haveSSE() bool {
+	_, _, _, d := cpuid(1)
+	return (d & (1 << 25)) != 0
+}