Add advanced button handler (#42)

* Add advanced button handler

* Rename pylgbst/extras/advancedbutton.py to examples/advancedbutton/advancedbutton.py

* Create README.md

examples/advancedbutton/README.md

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+### Advanced button
+
+This example shows how you can add additional functionallity to the move hub button. 
+
+It adds three new actions which you can use instead of the standard subscription to a button press:
+
+- Click - a single quick up/down press
+- Double click - a double up/down press, second click must occur within .5 secs of first one 
+- Long press - a press and hold on the button for > .7 secs 
+
+```python
+from pylgbst.hub import MoveHub
+from advancedbutton import AdvancedButton
+import time
+
+
+hub = MoveHub()
+b = AdvancedButton(hub)
+
+
+def clicked():
+    print("button clicked")
+
+
+def pressed():
+    print("button pressed")
+
+
+def doubleclicked():
+    print("button double clicked")
+
+
+b.click.subscribe(clicked)
+b.double_click.subscribe(doubleclicked)
+b.long_press.subscribe(pressed)
+
+time.sleep(120)
+```
+
+You can alter the timings using the two constants `DOUBLE_CLICK_TIME` and `LONG_PRESS_TIME`

examples/advancedbutton/advancedbutton.py

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+import time
+import threading
+
+DOUBLE_CLICK_TIME = 0.5
+LONG_PRESS_TIME = 0.7
+
+
+class AdvancedButton:
+    def __init__(self, hub):
+        self.state = 0
+        self.last_pressed = 0
+        self.press_time = None
+        self.hub = hub
+        self.hub.button.subscribe(self.pressed)
+        self.click = ButtonAction()
+        self.double_click = ButtonAction()
+        self.long_press = ButtonAction()
+
+    def pressed(self, state):
+        if state == 2:
+            return
+
+        press_time = time.time()
+
+        if state == 1:
+            self.state = 1
+            self.press_time = press_time
+            return
+
+        if state == 0 and self.state == 1:
+            self.state = 0
+            press_duration = press_time - self.press_time
+        else:
+            return
+
+        if press_duration > LONG_PRESS_TIME:
+            # long press
+            self.long_press.notify()
+            return
+
+        if (press_time - self.last_pressed) < DOUBLE_CLICK_TIME:
+            # double click
+            self.last_pressed = 0
+            self.double_click.notify()
+            return
+
+        # could be first of a double click, could be single click
+        self.last_pressed = press_time
+
+        def timeout():
+            time.sleep(DOUBLE_CLICK_TIME)
+            if self.last_pressed == press_time:
+                # not clicked while sleeping
+                # single click
+                self.click.notify()
+
+        threading.Thread(target=timeout).start()
+
+
+class ButtonAction:
+    def __init__(self):
+        self.subscribers = set()
+
+    def subscribe(self, callback):
+        self.subscribers.add(callback)
+
+    def unsubscribe(self, callback=None):
+        if callback in self.subscribers:
+            self.subscribers.remove(callback)
+
+    def notify(self):
+        for subscriber in self.subscribers.copy():
+            subscriber()