Fix trailing references causing memory leak

==== Fixed Issues ====

====  Tech Notes ====
This PR corrects a couple of small issues led to skills being left in memory when.
- Handler for `stop.mycroft` weren't removed from event emitter when
skill shut down. Now is added using `self.add_event()`
- registered intent list `self.events` created a circular reference that
python couldn't resolve a live so this is now deleted at shutdown
- Timers in scheduled skills weren't terminated properly. Now if the
timer is alive it will be joined

====  Documentation Notes ====
Registring event handlers should use `self.add_event` instead of
`self.emitter.on()` To make sure they are cleaned up when skill is
terminated.

==== Localization Notes ====
NONE - point to new strings, language specific functions, etc.

==== Environment Notes ====
NONE - new package requirements, new files being written to disk, etc.

==== Protocol Notes ====
NONE - message types added or changed, new signals, APIs, etc.

mycroft/skills/core.py

@@ -252,7 +252,7 @@ class MycroftSkill(object):
         self.stop_time = time.time()
         self.stop_threshold = self.config_core.get("skills").get(
             'stop_threshold')
-        self.emitter.on('mycroft.stop', self.__handle_stop)
+        self.add_event('mycroft.stop', self.__handle_stop, False)
 
     def detach(self):
         for (name, intent) in self.registered_intents:
@@ -304,7 +304,7 @@ class MycroftSkill(object):
         _intent_list = []
         _intent_file_list = []
 
-    def add_event(self, name, handler, need_self):
+    def add_event(self, name, handler, need_self=False):
         """
             Create event handler for executing intent
 
@@ -518,6 +518,7 @@ class MycroftSkill(object):
         # removing events
         for e, f in self.events:
             self.emitter.remove(e, f)
+        self.events = None  # Remove reference to wrappers
 
         self.emitter.emit(
             Message("detach_skill", {"skill_id": str(self.skill_id) + ":"}))

mycroft/skills/scheduled_skills.py

@@ -116,7 +116,11 @@ class ScheduledSkill(MycroftSkill):
 
     def shutdown(self):
         super(ScheduledSkill, self).shutdown()
+        # if timer method is running wait for it to complete
         self.cancel()
+        if self.timer and self.timer.isAlive():
+            self.timer.join()
+        self.timer = None
 
 
 class ScheduledCRUDSkill(ScheduledSkill):