Replace result type in str_to_f64

I have seen that you returned a tuple in the function str_to_f64. The first part of the tuple would be used as error message if the function doesn't succeed. However, I think in this case you could use Result<~[f64], &str> (you can find more information in http://static.rust-lang.org/doc/master/std/result/enum.Result.html).

By the way, I think this is a very nice project!