Dynamically make an option required based on a condition

```
Provide a way to specify that an option is only required if a certain condition 
is true.

.RequiredWhen(()=>true)
.RequiredWhen<User>(u=>u.Short == "System")

```

Original issue reported on code.google.com by `michael.praschl` on 21 Mar 2013 at 8:29
