add some unit conversion routines

	new file:   lib/POEx/Weather/OpenWeatherMap/Units.pm
Result; response_code attrib

	modified:   lib/POEx/Weather/OpenWeatherMap/Result.pm
Add Result::Current subclass, kill 'units' attrib

	modified:   lib/POEx/Weather/OpenWeatherMap.pm
	new file:   lib/POEx/Weather/OpenWeatherMap/Result/Current.pm
Request; hide units attrib

At the moment, always expect imperial and do conversions ourselves.

May change what we fetch later; either way, users shouldn't have to be
involved in the process ...

	modified:   lib/POEx/Weather/OpenWeatherMap/Request.pm