Ability to pass a simple address string to get a forecast

Hello,

Thanks for creating this wrapper!  I've added the ability for the user to enter a simple string that can be geocoded using Geopy instead of having to pass in an explicit lat and long.  Adding this feature of course adds another dependency.

I've also updated example.py to demonstrate how this would be used, and updated the test suite, however since it doesn't appear to actually call ```load_forecast```, it's not really getting tested.