Dijkstra's Algorithm

Hi, first time contributor here. I wrote up a straightforward program to solve Dijkstra's algorithm (http://rosettacode.org/wiki/Dijkstra%27s_algorithm). 

Do let me know if there are any issues :)
You do have to implement PartialOrd in order to implement Ord. It's written in the [declaration of the Ord trait](http://doc.rust-lang.org/std/cmp/trait.Ord.html): 

    pub trait Ord: Eq + PartialOrd // it means any type that implements ord
                                                   // also needs to implement Eq 
                                                   // and PartialOrd
That's what "deriving" does for you. It adds a basic implementation for a few "standard" traits without having to write one manually.