Expanded Evaluator to check for negative numbers

Problems encountered with the Evaluator class (as far as compiler
errors) were fixed and has been converted back over to using the enum
syntax_status rather than the string equivalents.

I've corrected a problem with the power operator to where it now handles
negative bases, negative exponents, and zero exponents.  The data types
used prevent useful output for the division operator and taking a base
to a negative number, but the functionality should still work.

test_input has been modified