mod. pep8.
rm. Pull of all users within a timeframe if `users` param is empty.
add. Raise an exception when users are not passed to process method.
mod. move UserMetricError Exception class outside of UserMetric class.
mod. added feature doc point for next version
add/mod. 'restrict' key to _param_types / pep8.
rm. existing restrict parameter modification for threshold and survival.
rm. start and end time params from method.
mod. synced _param_types definitions with UserMetric object attributes
add. 't' to default metric attributes.  This is the hours over which to measure any metric since an event (typically registration).
fix. ThresholdArgsClass constructor.
rm. _data_source_ attriubute. deprecated.
rm. date_[start|end] properties. deprecated.
mod. Centralized functionality to convert to mediawiki timestamps in user_metrics.utils.
fix/mod. varMappings to _param_types in metrics and preparation of results object in metrics manager.
mod. namespace formatting returns list
mod. JSON response to be pure: https://github.com/rfaulkner/E3_analysis/issues/54
add. an enumeration factory.
add. UserMetricPeriod classes to define time periods over which wuser metrics are computed.  Docs.
add. to _param_types new init parameter "period_type" that defines how time ranges are computed.