Finished implementing functionality of calculator

-Changed helper functions to be local functions
-Implemented error checking for bad inputs
Removed excess lunit files
Revert "Removed excess lunit files"

This reverts commit 01358b3a48480aeb1908824be8a4b657c85b58ce.
Removed error checking because

-I want conflict for you guys
Deleted arbitrary lunit
Updated README, test.lua

-Fixed specifications as depicted in the README
-Added bad input test for test.lua
Fixed bug in test.lua and changed default behavior in calculator

-If the operator does not exist, return error instead of nil
-Fixed bug in test.lua with pcall
Merge branch 'functionality' into documentation

Conflicts:
	calculator.lua