timestamp parameter has inconsistent behaviour

[davidmfrey]

The timestamp parameter in the params object doesn't behave as I would expect according to the documentation
https://www.mindmeld.com/docs/userguide/dm.html?highlight=timestamp#params
It is listed as a unix timestamp which should be represented in seconds, however the handling of it attempts to differentiate between a seconds and milliseconds measurement.
https://github.com/cisco/mindmeld/blob/master/mindmeld/ser.py#L151
The API definition should include the units and the behaviour should follow the definition.
The current method of checking the number of digits in the timestamp is not robust. This causes confusing behaviour when using timestamps older than 2001.