feature request: query_metric(start/end params) to support native Python datetime #43

Python prides itself for English like/expressive syntax. Currently using Java-esque epoch time for start and end time range in query_metric(..., start=<start_timestamp>, end=<end_timestamp>) seems to a bit awkward to work with (at least in 2.7). I have to add the following code to get metrics within the last 5 minutes.

def unix_time_millis(dt):
    return '{:.0f}'.format((dt - epoch).total_seconds() * 1000)
...
now = datetime.utcnow()
end = unix_time_millis(now)
start = unix_time_millis(now - timedelta(minutes=5))
data = client.query_metric(MetricType.Counter, metric_id, start=start, end=end)

It would be nice if query_metric() can support native Python datetime, time, timedelta.