Fix #540 : Added more tests to nth-prime and altered example code

[gavinhenderson]

The test execute time has gone from 2.975s with the old example code and the old tests to 3.200s with the new tests and the new example code.

[matthewmorgan]

@gavinhenderson looks great. I took a similar approach with my solution a while back! Though you can see I was lazy and imported my prior sieve function...

I appreciate the fact that you commented explaining the magical math. It might be improved slightly by referring to a citation that explains the formula. By way of example, here's that old solution I mentioned:

import sieve from './sieve';  //my Sieve class from earlier

export default class Prime{

  // see http://en.wikipedia.org/wiki/Prime_number_theorem#Approximations_for_the_nth_prime_number
  nth(n) {
    if (n < 1) throw new Error("Prime is not possible");
    let approximateLimit = n * Math.log(n) + n * Math.log(Math.log(n));
    approximateLimit < 15 ? approximateLimit = 15 : {};
    return sieve(approximateLimit).primes[n - 1];
  }
}

Would you mind making that comment change? Otherwise this PR looks good to me.

[gavinhenderson]

@matthewmorgan Thats the link added now

[matthewmorgan]

@gavinhenderson awesome-- thank you.

Can I ask you to squash your commits to one for this PR? Please let me know if you need assistance with this, I'd be happy to help.

[gavinhenderson]

@matthewmorgan So I think I might have done it?

[matthewmorgan]

Nicely done. Thanks for your contribution!