asyncio: sync with Tulip

- Tulip issue 185: Add a create_task() method to event loops. The create_task()
  method can be overriden in custom event loop to implement their own task
  class. For example, greenio and Pulsar projects use their own task class. The
  create_task() method is now preferred over creating directly task using the
  Task class.
- tests: fix a warning
- fix typo in the name of a test function
- Update AbstractEventLoop: add new event loop methods; update also the unit test

Lib/asyncio/events.py

@@ -200,6 +200,10 @@ class AbstractEventLoop:
         """Return whether the event loop is currently running."""
         raise NotImplementedError
 
+    def is_closed(self):
+        """Returns True if the event loop was closed."""
+        raise NotImplementedError
+
     def close(self):
         """Close the loop.
 
@@ -225,6 +229,11 @@ class AbstractEventLoop:
     def time(self):
         raise NotImplementedError
 
+    # Method scheduling a coroutine object: create a task.
+
+    def create_task(self, coro):
+        raise NotImplementedError
+
     # Methods for interacting with threads.
 
     def call_soon_threadsafe(self, callback, *args):