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.

mage.go

@@ -0,0 +1,11 @@
+// +build ignore
+
+package main
+
+import (
+	"os"
+
+	"github.com/magefile/mage/mage"
+)
+
+func main() { os.Exit(mage.Main()) }