Golang runtime (#125)

Minimal golang runtime.  

Functions are built as Go plugins, and the plugin is uploaded to fission.

See `environments/go/README.md` for instructions and `examples/go` for a usage example.

For now, we're re-using the 'code' field of the function to store the binary plugin. With v2 environments, this will be stored separately as a binary package.

examples/go/hello.go

@@ -0,0 +1,10 @@
+package main
+
+import (
+	"net/http"
+)
+
+func Handler(w http.ResponseWriter, r *http.Request) {
+	msg := "Hello, World!"
+	w.Write([]byte(msg))
+}