Added FirstOrDefault signature in IEnumerableSignatures interface. Shoul...

Hello, I had to add FirstOrDefault signature in IEnumerableSignatures for my needs, thought it'd be nice to let you know. (Makes me able to OrderBy(Person.Addresses.FirstOrDefault().Street for example).

I also think other basics signatures like Single, First, SingleOrDefault could be added in IEnumerableSignatures, up to you.