Convert from the retired ExFSWatch to FileSystem

Initial working commit. Use the new GenServer only API.

lib/watcher.ex

@@ -1,14 +1,21 @@
 defmodule Watcher do
-  use ExFSWatch, dirs: ["lib/koans"]
+  use GenServer
 
-  def start_link do
-    start()
+  def start_link(args) do
+    GenServer.start_link(__MODULE__, args)
   end
 
-  def callback(file, events)  do
+  def init(args) do
+    {:ok, watcher_pid} = FileSystem.start_link(dirs: ["lib/koans"])
+    FileSystem.subscribe(watcher_pid)
+    {:ok, %{watcher_pid: watcher_pid}}
+  end
+
+  def handle_info({:file_event, watcher_pid, {path, events}}, %{watcher_pid: watcher_pid}=state) do
     if Enum.member?(events, :modified) do
-      file |> normalize |> reload
+      path |> normalize |> reload
     end
+    {:noreply, state}
   end
 
   defp reload(file) do