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.
add. bytes added now computes relative to different user periods.
fix. test runner dependencies.
mod. utilize UserMetricPeriod in edit count.
add. test for edit count metric.
add. method build_namedtuple to dynamically build namedtuple type objects
add. Multiprocessing to edit count metric.
mod. _param_types should store type casting methods rather than strings representing them.