Fixed let block

Prior to comitting code at 3am, its best to run your test suite to make
sure everything still passes as expected... Especially when you are
experimenitng with different implementations.

RookieMistake.new("run your suite before you commit")
Negative numbers identified in ArgumentError

Added functionality to include negative numbers in ArgumentError if
string includes negative numbers.

Refactored, #add, #validate methods and added #negative?,
\#multiple_delimiters?, #collect_negatives and #prepare_value_array in
an attempt to keep each method's responsibility singular.
Fixed typo in #mulitple_delimiters?
Refactor

Refactored to include initialize method. By using an initialize method
and passing our string directly StringCalculator and adding a
attr_reader I was able to dry up the code and make it much more pleasing
to read.