Added is-div? and tests

* Added is-div? function to see if a number is evenly divisible by
another
* Added tests for is-div? function
Optimized the code and completed testingg

* Refactored code to use is-div?
* Implemented fully-divide
* Tested fully-divide
* Implemented find-prime-factors
* Tested find-prime-factors
* Updated problem3 to use find-prime-factors