Unexpected panics due to unwrap

[Xau9472]

I found two panic cases due to calling unwrap on None.

let datetime = Local::now();
datetime.sub(Months::new(312345123)); // first panic
datetime.sub(Days::new(312345123)); // second panic

It might be better to have panic msgs (e.g., with expect) and mention them in the documentation.

[pitdicker]

Thank you for the report. Changed to expect and added documentation in #1316.