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.

environments/go/context/context.go

@@ -0,0 +1,10 @@
+package context
+
+type (
+	Context map[string]interface{}
+)
+
+func New() Context {
+	ctx := make(map[string]interface{})
+	return ctx
+}