Normal/Gaussian random number generation for D

[Joseph Rushton Wakeling]

Just to update everyone, Jerro and I have been knocking around a few discussions 
and code tweaks on GitHub:
https://github.com/WebDrake/phobos/pull/1#issuecomment-10319570

If anyone wants to comment or give feedback, please feel free.  It's surely 
useful that we get as many concerns covered and addressed before submitting an 
official Phobos pull request!