fixing bad import

Looks like you forgot this when you did your big rename? This is what you would see when you tried to run the example without this change:

[master:?][bwilliams@bobot:~/Projects/python-forcast.io]
[17:27:31] $ python example.py
Traceback (most recent call last):
  File "example.py", line 1, in <module>
    import forecastio
  File "/Users/bwilliams/Projects/python-forcast.io/forecastio/__init__.py", line 1, in <module>
    from api import load_forecast
  File "/Users/bwilliams/Projects/python-forcast.io/forecastio/api.py", line 5, in <module>
    from forecastio import Forecast
ImportError: cannot import name Forecast