Extension method to convert an Option<T> to a Nullable<T>

[atifaziz]

This PR helps with #13 when a conversion to Nullable<T> is needed.

It also helps with the example presented in issue #17, which can be written as:

var d = Option.Some(new DateTime(2010, 12, 31));
var s = string.Format(new CultureInfo("de-DE"), "{0:D}", d.ToNullable());
Console.WriteLine(s);

instead of:

var d = Option.Some(new DateTime(2010, 12, 31));
var s = string.Format(new CultureInfo("de-DE"), "{0:D}", d.Match(v => v, () => default(DateTime?)));
Console.WriteLine(s);

That is, of course, while #17 is open.

The method being added by this PR is called ToNullable() instead of ValueOrNull() because you cannot overload on generic type parameter constraints and if ValueOrNull() is subsequently added then that would only apply to Option<T> where T is a reference.

[nlkl]

Closing issue, as this has now been implemented.