Refactor repository layout and convert build system to Mage.

This commit implements a massive refactor of the repository, and
moves the build system over to use Mage (magefile.org) which should
allow seamless building across multiple platforms.

vendor/github.com/magefile/mage/magefile.go

@@ -0,0 +1,94 @@
+//+build mage
+
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"time"
+
+	"github.com/magefile/mage/sh"
+)
+
+// Runs "go install" for mage.  This generates the version info the binary.
+func Install() error {
+	ldf, err := flags()
+	if err != nil {
+		return err
+	}
+
+	name := "mage"
+	if runtime.GOOS == "windows" {
+		name += ".exe"
+	}
+	gopath, err := sh.Output("go", "env", "GOPATH")
+	if err != nil {
+		return fmt.Errorf("can't determine GOPATH: %v", err)
+	}
+	paths := strings.Split(gopath, string([]rune{os.PathListSeparator}))
+	bin := filepath.Join(paths[0], "bin")
+	// specifically don't mkdirall, if you have an invalid gopath in the first
+	// place, that's not on us to fix.
+	if err := os.Mkdir(bin, 0700); err != nil && !os.IsExist(err) {
+		return fmt.Errorf("failed to create %q: %v", bin, err)
+	}
+	path := filepath.Join(bin, name)
+
+	// we use go build here because if someone built with go get, then `go
+	// install` turns into a no-op, and `go install -a` fails on people's
+	// machines that have go installed in a non-writeable directory (such as
+	// normal OS installs in /usr/bin)
+	return sh.RunV("go", "build", "-o", path, "-ldflags="+ldf, "github.com/magefile/mage")
+}
+
+// Generates a new release.  Expects the TAG environment variable to be set,
+// which will create a new tag with that name.
+func Release() (err error) {
+	if os.Getenv("TAG") == "" {
+		return errors.New("MSG and TAG environment variables are required")
+	}
+	if err := sh.RunV("git", "tag", "-a", "$TAG"); err != nil {
+		return err
+	}
+	if err := sh.RunV("git", "push", "origin", "$TAG"); err != nil {
+		return err
+	}
+	defer func() {
+		if err != nil {
+			sh.RunV("git", "tag", "--delete", "$TAG")
+			sh.RunV("git", "push", "--delete", "origin", "$TAG")
+		}
+	}()
+	return sh.RunV("goreleaser")
+}
+
+// Remove the temporarily generated files from Release.
+func Clean() error {
+	return sh.Rm("dist")
+}
+
+func flags() (string, error) {
+	timestamp := time.Now().Format(time.RFC3339)
+	hash := hash()
+	tag := tag()
+	if tag == "" {
+		tag = "dev"
+	}
+	return fmt.Sprintf(`-X "github.com/magefile/mage/mage.timestamp=%s" -X "github.com/magefile/mage/mage.commitHash=%s" -X "github.com/magefile/mage/mage.gitTag=%s"`, timestamp, hash, tag), nil
+}
+
+// tag returns the git tag for the current branch or "" if none.
+func tag() string {
+	s, _ := sh.Output("git", "describe", "--tags")
+	return s
+}
+
+// hash returns the git hash for the current repo or "" if none.
+func hash() string {
+	hash, _ := sh.Output("git", "rev-parse", "--short", "HEAD")
+	return hash
+}