Fibonacci impl

Added classic recursive, tail recursive and iterative implementation of the Fibonacci sequence
The thing is that I am not sure what other use case advance is meant for. Even [the example in the docs](http://doc.rust-lang.org/std/iter/trait.Iterator.html#tymethod.advance) uses it as a for_each.
Since advance itself only returns a bool, the closure inside it must have some type of side-effect to make it useful. 

By the way Eric Lippert [arguments against adding a ForEach](http://blogs.msdn.com/b/ericlippert/archive/2009/05/18/foreach-vs-foreach.aspx) to C# version of iterators. I thought it would be an interesting read, because all the arguments he offers are pretty general and could apply to Rust as well.
I'm not linking his post because I am against adding for_each to Rust (I am using advance so I would happily use for_each if it existed) but I thought you might find it interesting.