Add IsPrime method

Created IsPrime method to determine if a number is prime or not, also
created associated tests
Re-factored test

Modified the IsPrime test to use a map to make it easier to test for new
values.  Also makes the test more readable.
Modified PrimeFactors method

The PrimeFactors method has been modified to better use maps as per the
Go language, this has removed the need for the extra function as the
language will sort itself out