Get will now return a (*Forecast, error) pair and units are configurable

I removed the calls to log.Fatal, since they cause a panic - I don't believe this is warranted in a library, and can be caused by minor problems such as network failure.

Additionally, InsecureSkipVerify is disabled - forecast.io seems to have a proper X509 certificate, so this should not be needed anymore.

Finally, I added two consts, CA and SI that can be passed as an argument to Get to set the units in the response.