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/README.md

@@ -0,0 +1,29 @@
+# Fission: Go Environment
+
+This is the Go environment for Fission.
+
+It's a Docker image containing a Go 1.8rc3 runtime, along with a dynamic loader.
+
+## Build this image
+
+```
+docker build -t USER/go-runtime . && docker push USER/go-runtime
+```
+
+## Using the image in fission
+
+You can add this customized image to fission with "fission env
+create":
+
+```
+fission env create --name go-runtime --image USER/go-runtime
+```
+
+Or, if you already have an environment, you can update its image:
+
+```
+fission env update --name go-runtime --image USER/go-runtime   
+```
+
+After this, fission functions that have the env parameter set to the
+same environment name as this command will use this environment.