Add Environment v2 Builder (#298)

Build server and client for environments.

This build server will run inside environment builders and invoke environment-specific scripts for doing builds. It provides a uniform language-independent API for builds to the fission build manager.

The change also includes an implementation of a builder for Python -- this "builder" acts on requirements.txt and runs `pip install` to collect all function deps.

builder/cmd/Dockerfile

@@ -0,0 +1,5 @@
+FROM alpine:3.4
+
+ADD builder /
+
+EXPOSE 8000

builder/cmd/build.sh

@@ -0,0 +1,2 @@
+#!/bin/sh
+GOOS=linux GOARCH=386 go build -o builder .

builder/cmd/main.go

@@ -0,0 +1,42 @@
+/*
+Copyright 2016 The Fission Authors.
+
+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 main
+
+import (
+	"log"
+	"net/http"
+	"os"
+
+	builder "github.com/fission/fission/builder"
+)
+
+// Usage: builder <shared volume path>
+func main() {
+	dir := os.Args[1]
+	if _, err := os.Stat(dir); err != nil {
+		if os.IsNotExist(err) {
+			err = os.MkdirAll(dir, os.ModeDir|0700)
+			if err != nil {
+				log.Fatalf("Error creating directory: %v", err)
+			}
+		}
+	}
+	builder := builder.MakeBuilder(dir)
+	mux := http.NewServeMux()
+	mux.HandleFunc("/", builder.Handler)
+	http.ListenAndServe(":8000", mux)
+}