postfix

postfix is a postfix math notation calculator. Postfix notation are math expressions where the operators follow the operands. There is thus no need for braces as the order of the operators enforce the order of operations.

Example

```rs extern crate postfix; use postfix::PostfixCalculator;

let calculator = PostfixCalculator::new(); println!("result: {:?}", calculator.evaluate(&vec!["4", "2", "+"])); // result: 6.0 ```