Allow sub-second timestamp resolution

In my application of using btmon.py with a Brultech ECM-1240 connected over USB serial to a Netgear R6300v2 running FreshTomato, I was having severe problems with captured data samples being queued out of order. This would consequently cause Grafana graphs of the data to be very spiky and full of dropouts.

Allowing btmon.py to keep the fractional seconds has eliminated this problem.

bin/btmon.py

@@ -1334,7 +1334,7 @@ def mkts(seconds):
     return time.strftime('%Y-%m-%dT%H:%M:%SZ', time.gmtime(seconds))
 
 def getgmtime():
-    return int(time.time())
+    return time.time()
 
 def cleanvalue(s):
     """ensure that values read from configuration file are sane"""