Fixed unit tests broken by the pull request.
Added a fix for the odd requirement that timestamps for get_metric_data_points() need to be in milliseconds. GitHub #176
autoscale scaling policy desiredCapacity support

Add a "desired_capacity" parameter to all of the ScalingPolicy
manipulation functions, and use it to set the "desiredCapacity"
attribute on the json object for created and updated policies.
update the add_policy methods too.