Primes Factorization

[vitaly-t]

I added initial implementation of primeFactors function today. It works fine, for most of the practical cases. But it is very basic, and may get stuck on some interesting large numbers, like 2**50 - 2.

I will work on improving its performance when I get a chance. In the meantime, if somebody wants to contribute - PR-s are welcome!

[vitaly-t]

The function has been fully finished, and it is mighty fast :)

[ishandutta2007]

@vitaly-t Can you add it to benchmarks page

[vitaly-t]

If I get a chance to look at it again. In the meantime, I remember looking at a number of implementations in JavaScript, and mine was the fastest one that I could find, i.e. I took the fastest implementation out there, and then optimized it thoroughly.