Use a single worker for status refreshes

There still appear to be crashes occurring when
using libgit2 from multiple workers at the same time.

So only start a new status worker once the current one
completes if a refresh was requested while a worker was
running.

Closes #367

src/stdlib/task.coffee

@@ -1,3 +1,6 @@
+_ = require 'underscore'
+EventEmitter = require 'event-emitter'
+
 module.exports =
 class Task
   aborted: false
@@ -49,3 +52,6 @@ class Task
     @abort()
     @worker?.terminate()
     @worker = null
+    @trigger 'task-completed'
+
+_.extend Task.prototype, EventEmitter