Adding MetricData class.

The ``get_metric_data`` method currently returns the list of DataPoints returned from CloudWatch.  The problem with this is that in the case where you have not supplied an explicit ``period`` argument, the ``period`` will be calculated by ``skew`` to be as small as possible and still return no more datapoints than are allowed by CloudWatch.  But in this case, the end user has no idea what the ``period`` value actually is and in cases where the ``statistic`` is ``Sum`` you really need to know the period to compute the actual per/second values.

So, this PR adds a ``MetricData`` class that is returned by ``get_metric_data``.  This includes the data itself, in a ``data`` attribute and the period, in a ``period`` attribute.

This is an incompatible, breaking change.