Lionel

Hi Everyone. This is my first attempt at the On Screen Calculator. My commits are not small, and I apologize for that Zee. The only functionality that I see that throws an error is clicking on operators more than once before clicking on another number. I don't think some of the edge cases I addressed were necessary, but since I'm building these applications for my portfolio I'd like it to be fairly clean.

Thank you for your help.
To be honest, I'd probably have `operators` be a child of Calculator:

```
Calculator.operators = {
  '+': function(a,b) {
    return a + b;
  },
  etc...
}
```