stable and consistent RNG C++ class

[rmandelb]

Per our discussion on the list, we want to make sure that the RNG we use (a) is open-source and (b) can be relied on to always produce the same random sequence given the same initial seed. Gary proposed and is implementing and documenting a solution that should fit those two requirements, so this issue is assigned to him (and is connected to #1 and #3, but I'm listing it separately as it is also a solution to other long-term problems).

Barney volunteered to make unit tests, which will be a separate issue.

[rmandelb]

Redundant with #1, closing.