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/mg/runtime.go

@@ -0,0 +1,36 @@
+package mg
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+// CacheEnv is the environment variable that users may set to change the
+// location where mage stores its compiled binaries.
+const CacheEnv = "MAGEFILE_CACHE"
+
+// verboseEnv is the environment variable that indicates the user requested
+// verbose mode when running a magefile.
+const verboseEnv = "MAGEFILE_VERBOSE"
+
+// Verbose reports whether a magefile was run with the verbose flag.
+func Verbose() bool {
+	return os.Getenv(verboseEnv) != ""
+}
+
+// CacheDir returns the directory where mage caches compiled binaries.  It
+// defaults to $HOME/.magefile, but may be overridden by the MAGEFILE_CACHE
+// environment variable.
+func CacheDir() string {
+	d := os.Getenv(CacheEnv)
+	if d != "" {
+		return d
+	}
+	switch runtime.GOOS {
+	case "windows":
+		return filepath.Join(os.Getenv("HOMEDRIVE"), os.Getenv("HOMEPATH"), "magefile")
+	default:
+		return filepath.Join(os.Getenv("HOME"), ".magefile")
+	}
+}