get_activities: accept either a timestamp or a datetime object for the `since` parameter.
get_activities: accept either a timestamp or a datetime object for the `since` parameter.

Because the timestamp expected by smashrun is a bit non-standard, it
might be helpful to allow users to generate their own timestamp (if they
know what they are doing) and pass that in instead of relying on us to
attempt to convert it properly.

campbellr/smashrun-client