String Calculator Kata

The following is a TDD Kata, an exercise in coding, refactoring and test-first, that you should apply daily for at least 15-30 minutes.

Before you start

Try not to read ahead.
Do one task at a time. The trick is to learn to work incrementally.
Make sure you only test for correct inputs. There is no need to test for invalid inputs for this kata.

Create a simple String calculator with a method int add(String numbers).

The string argument can contain 0, 1 or 2 numbers, and will return their sum (for an empty string it will return 0) for example "" or "1" or "1,2".
Start with the simplest test case of an empty string and move to 1 and two numbers.
Remember to solve things as simply as possible so that you force yourself to write tests you did not think about.
Remember to refactor after each passing test.

Allow the add() method to handle an unknown amount of numbers.

Allow the add() method to handle new lines between numbers (instead of commas).

the following input is ok: "1\n2,3" (will equal 6)
the following input is NOT ok: "1,\n" (not need to prove it - just clarifying)

Support different delimiters: to change a delimiter, the beginning of the string will contain a separate line that looks like this:

"//[delimiter]\n[numbers...]"

For example "//;\n1;2" should return 3 where the default delimiter is ';'.

The first line is optional.
All existing scenarios should still be supported.

Calling add() with a negative number will throw an exception "negatives not allowed" - and the negative that was passed.

For example add("1,4,-1") should throw an exception with the message "negatives not allowed: -1".

If there are multiple negatives, show all of them in the exception message.

Numbers bigger than 1000 should be ignored, so adding 2 + 1001 = 2

Use whatever language and frameworks you want. Use something that you know well.
Provide a README with instructions on how to compile and run the application.

IMPORTANT: Implement the requirements focusing on writing the best code you can produce.

CODE SUBMISSION: Add the code to your own Github account and send us the link.

Credits to Roy Osherove for the original idea.