put in references for JAGS and STAN

[davidwhogg]

These need URLs and references. I am putting this on @dfm although I feel a little bit responsibility-shirking when I do that.

[pirg]

Hi,
You probably meant to remove Stan and keep JAGS in this commit. Stan doesn't do any Gibbs sampling, only the only sampling they use is HMC (by default through NUTS)
Pierre

[davidwhogg]

whoops; reopening in case we have a mistake in our text or implications

[davidwhogg]

Also see email by @bob-carpenter on 2017-11-01