Fix to improve accuracy of timer

Timer was not accurate as it was depending on the asyncio.sleep function which is most liklely not exactly one second depending on other functions running in parallel.

Now the timer is comparable to cbpi3 where it runs until an end time is reached.

This is to address issue #66 and has been tested

cbpi/api/timer.py

@@ -13,27 +13,29 @@ class Timer(object):
         self._callback = on_done
         self._update = on_update
         self.start_time = None
+        self.end_time = None
     
     def done(self, task):
         if self._callback is not None:
             asyncio.create_task(self._callback(self))
 
     async def _job(self):
-        self.start_time = time.time()
-        self.count = int(round(self._timemout, 0))
+        self.start_time = int(time.time())
+        self.end_time = self.start_time + int(round(self._timemout, 0))
+        self.count = self.end_time - self.start_time
         try:
             while self.count > 0:
-                self.count -= 1
+                self.count = (self.end_time - int(time.time()))
                 if self._update is not None:
                     await self._update(self,self.count)
                 await asyncio.sleep(1)
         except asyncio.CancelledError:
-            end = time.time()
+            end = int(time.time())
             duration = end - self.start_time
             self._timemout = self._timemout - duration
 
     async def add(self, seconds):
-        self.count = self.count + seconds
+        self.end_time = self.end_time + seconds
 
     def start(self):
         self._task = asyncio.create_task(self._job())