Fuzz test large integers

[bspeice]

Related to #21. That issue deals with panics on parsing integers, this deals with a separate class of issues.

The following test case panics on an out-of-range date:

#[test]
fn another_large_int() {
    let parse_result = parse("1412409095009");
    assert!(parse_result.is_err());
    println!("{:?}", parse_result);
}