Refactor converters to use abstract class

[treffynnon]

The convert() and reverse() functions are algorithmically the same so they can be factored out of each converter (https://github.com/treffynnon/Navigator/blob/master/lib/Treffynnon/Navigator/Distance/Converter/MetreToFurlong.php) and into the abstract class (https://github.com/treffynnon/Navigator/blob/master/lib/Treffynnon/Navigator/Distance/Converter/ConverterAbstract.php) they extend.