Add a heartbeat method to the py.Stream class (more below)

There are a number of ways to implement this, but this is the most
similar to how the rest of the library works. The best approach would
be to return messages from the server along with messages generated
in the library to give the user as much information as possible.

Author: theengineear

plotly/plotly/plotly.py

@@ -416,6 +416,21 @@ def __init__(self, stream_id):
         self.stream_id = stream_id
         self.connected = False
 
+    def heartbeat(self, reconnect_on=(200, '', 408)):
+        """Keep stream alive. Streams will close after ~1 min of inactivity.
+
+        If the interval between stream writes is > 30 seconds, you should
+        consider adding a heartbeat between your stream.write() calls like so:
+        >>> stream.heartbeat()
+
+        """
+        try:
+            self._stream.write('\n', reconnect_on=reconnect_on)
+        except AttributeError:
+            raise exceptions.PlotlyError("Stream has not been opened yet, "
+                                         "cannot write to a closed connection. "
+                                         "Call `open()` on the stream to open the stream.")
+
     def open(self):
         """Open streaming connection to plotly.