implement the calculator and a test

[rmd6502]

Hi Kaleb,
I've implemented a few calculators in my day, so I thought I'd help a Maker out.
There's also a test, that should run in a Linux or mac command line by cd'ing to the test directory, then make and ./testCalculator. Since I don't have your Arduino setup I couldn't test it in situ, but it should work with minimal modification.

[KalebClark]

I have no idea how I am just now seeing this, but this is awesome!

(disclaimer: C is not my strong point, I get by, and am learning!)

I just pulled these additions and received an error on the index() function. I tried to track that down and so far what I have found is it is in a "strings.h" but not "string.h" as included. I looked for any reference to that function in arduino and so far coming up empty.

Do you have any reference for that in arduino code? I can keep looking, but wanted to check with you first.

Thanks,
-Kaleb