isprime.py : Added timing framework

isprime.py has been made into a standalone file/module. We will call isprime from other files/functions by importing it. Look at driver.py on how to use it.

to isprime.py itself, a timing framework has been added. Run isprime.py to see how long it takes to calculate primes to 1 million on your machine. At this point we can call isprime good and move on. We will revisit to make it better in the future.