Replace return 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).

I have used pattern matching to check if the returned value is an error, but this isn't strictly necessary. If you think it is too verbose you can instead use `result.is_err()` combined with `result.unwrap()` (for Ok values) or `result.unwrap_err()` (for Err values).

By the way, I think this is a very nice project!