Fibonacci impl

Added classic recursive, tail recursive and iterative implementation of the Fibonacci sequence
tester is just a helper function, I use it here in fib_values() here:
```
    let fns=vec![fib_recursive, fib_tail_recursive, fib_iterative];
    fns.move_iter()
        .advance(|f| tester(f));
```
[advance](http://doc.rust-lang.org/std/iter/trait.Iterator.html#tymethod.advance) needs tester to return a bool