Add ConfigurableMeter Class

As mentioned in #446, the purpose of this is to add a version of the Meter class which allows custom EWMA windows to be specified.  The new ``ConfigurableMeter`` class does that and now ``Meter`` extends that class.

Because the ``Metered`` interface explicitly specifies 1, 5, and 15 minute rates, the API for this is a bit of an awkward fit.  If there are any changes I can make to resolve this, please let me know.

Thanks, and I look forward to feedback!