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/sh/helpers.go

@@ -0,0 +1,16 @@
+package sh
+
+import (
+	"fmt"
+	"os"
+)
+
+// Rm removes the given file or directory even if non-empty. It will not return
+// an error if the target doesn't exist, only if the target cannot be removed.
+func Rm(path string) error {
+	err := os.RemoveAll(path)
+	if err == nil || os.IsNotExist(err) {
+		return nil
+	}
+	return fmt.Errorf(`failed to remove %s: %v`, path, err)
+}