Improve stats when progressing too quickly

Do not update stats if time between data points is less than
`time_threshold`.  This should fix the issue reported in #24.

progress/__init__.py

@@ -26,13 +26,15 @@ __version__ = '1.2'
 
 class Infinite(object):
     file = stderr
-    sma_window = 10
+    sma_window = 10         # Simple Moving Average window
+    time_threshold = 0.1    # Minimum distance between data points (in seconds)
 
     def __init__(self, *args, **kwargs):
         self.index = 0
         self.start_ts = time()
         self._ts = self.start_ts
         self._dt = deque(maxlen=self.sma_window)
+        self._pending = 0
         for key, val in kwargs.items():
             setattr(self, key, val)
 
@@ -65,9 +67,13 @@ class Infinite(object):
     def next(self, n=1):
         if n > 0:
             now = time()
-            dt = (now - self._ts) / n
-            self._dt.append(dt)
-            self._ts = now
+            dt = now - self._ts
+            if dt < self.time_threshold:
+                self._pending += n
+            else:
+                self._dt.append(dt / (n + self._pending))
+                self._ts = now
+                self._pending = 0
 
         self.index = self.index + n
         self.update()