Faster primes

I wrote a new implementation of Prime::EratosthenesGenerator/EratosthenesSieve.  It's a simple implementation of a segmented sieve of Eratosthenes but in my tests has performed much faster than the library version (see graphs).

It has the same interface and passes the same unit test as the original.

![chart_1](https://f.cloud.github.com/assets/2107646/724561/beaeaa0e-e038-11e2-823e-2bf909ef8708.png)

![chart_2](https://f.cloud.github.com/assets/2107646/724562/c886d434-e038-11e2-8afc-08416b9cfeb8.png)