[weather.py] Total overhaul - switched from Yahoo to OWM

So I've now finished the weather.py overhaul and I have to say it is way better than the Yahoo version.

So what's different now?
First of all, the wind directions and air pressures are no longer messed up like they were on the Yahoo version (air pressure and wind direction were plain wrong).
On top of that this module features all the same things as the yahoo version does and more.

Here's a comparison:
01:09:59 < Meicceli> .wea Helsinki
01:10:00 < Willie> Yahoo! Weather - Helsinki, FI: Light Rain, 8°C (46°F), 29in (982mb), Gentle breeze 3.6m/s (→)
01:10:01 < weather> Helsinki 00:19:19 - Temperature: 8.5°C (47.3°F); Wind: 1.03m/s (Northeast); Gust: 3.08m/s; Humidity: 91%; Air pressure: 998hPa; light intensity shower rain; Sunrise: 04:49; Sunset: 21:44; (openweathermap.org)

I've tested the module well enough to know that it works as it is supposed to work. I have been able to produce no errors with the module.

The usage is the same as it is with the Yahoo version. You can set your location with `.setlocation` (though `.setwoeid` has been removed) and you can request weather with `.wea [location]` or `.weather [location]`

Note: On MySQL there might be problems with storing locations that have non-ascii characters in their English name (e.g. "Ylöjärvi" which is a city in Finland that has no English translation)