added an option to view forecastio irradiance data

Forecast.io has a beta option to view irradiance data.  I'd like be able to use this, so I added the parameter solar to the method: forecastio.api.load_forecast

The way to view this data would be:

import forecastio
from forecastio.utils import PropertyUnavailable
forecast = forecastio.load_forecast('%^&!@*#(@&#@()*$#',37.8267,-122.423, solar=True)
byHour = forecast.hourly()
for hourlyData in byHour.data:
        try:
            print hourlyData.time, hourlyData.solar
        except PropertyUnavailable: pass

This irradiance data is only available during daylight hours, which is why you we need to skip occasionally.