Changing Forecast::IO to ForecastIO to avoid naming collisions

As discussed in an earlier comment, here's the pull request with the namespacing change.

Generally I feel gems should have namespaces matching their names, and make a reasonable attempt to avoid collisions with commonly named objects in others' code. Granted, this change will require a serious bump to the version number - probably to 2.0.

If you don't want to merge this in, keen to hear reasons why :)