add .gitignore rules for Intellij Idea files. Updated build.sbt
add UnitSpec as base class for unit tests with FunSpec an Matchers, Add failing test for StringCalculator
add test case and implementation on empty string.
add test case and implementation on single number.
add test case and implementation two numbers delimited by a comma.
add test case and implementation for using newline character as delimiter too.
add test case and implementation for user specified delimiter.
add test case and implementation for throwing exception when an negative number is in the input.
add test case and implementation for showing negative numbers in the exception message.