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.
Y no tests for `Calculator.calculate`?! That's the meat of your application! Plus if you use a calculation that adds, multiplies, divides, etc. you don't need to test each of those functions independently (unless you want to)