Fibonacci impl

Added classic recursive, tail recursive and iterative implementation of the Fibonacci sequence
What do you think about using a nested loop instead of making a vector?

```
for (f, desc) in fns.move_iter() {
    for fib in range(0u64, 10).map(|i| f(i)) {
        println!("{}:\n {}\n", desc, r);
    }
}
```