Merge PR #454 - add simple calculator watch face

Merge PR #454 - add simple calculator watch face

Adds a simple calculator watch face to the sensor watch
capable of addition, subtraction, multiplication, division
and exponentiation.

Reviewed-by: Joey Castillo <joeycastillo@utexas.edu>
Reviewed-by: Matheus Afonso Martins Moreira <matheus@matheusmoreira.com>
Tested-by: Joey Castillo <joeycastillo@utexas.edu>
Tested-on-hardware-by: mcguirepr89 <mcguirepr89@gmail.com>
GitHub-Pull-Request: https://github.com/joeycastillo/Sensor-Watch/pull/454