Step 5

* Looking at this new requirement I don't think a String array is now
the best thing. So this is a good chance to take stock and reevalulate.
* It seems that we have two things going on here, a Adder and Parser.
Maybe it was a mistake to assume this was a calculator.
* Decided to leave tests for now and refactor
* Moving the tokeniser block to a new NumberParser which will parse the
input into a List<Integer> and refactor existing tests to handle this.
* First things first lets refactor and keep the String[]
* Moved tokenise to new class NumberParser and added a String[]
getNumbers(input).
* All tests still pass on the StringCalculator.
* Copied tests from StringCalculator to NumberParser refactoring as I go
to have both classes covered correctly.
* Used tests in StringCalculator as basis for new tests in NumberParser.
* Deleted tests in the StringCalculatorTest and only test for "", one
number, which isn't really a special case but still feels right, 2
different numbers and 2 the same.