Get now returns a (*Forecast, error) pair

This seems like good practice. As it was, the code would panic in case
of - for example - network problems, which is not the nicest thing to do
in a library.

Additionally, InsecureSkipVerify has been set to false.
Make units configurable
Merge pull request #1 from tdubrownik/master

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