Feature/stagnant model bugs vol2

Added BUGS volume 2 example model stagnant in both discrete (marginalized out) and continuous parameterizations.

Remove branch after merge.

I didn't know how to add tests for expected values.  For model stagnant2.stan, the expected  values from BUGS are as follows (the Stan model gets these right):

```
alpha 0.537
beta[1] -0.4184
beta[2] -1.014
sigma 0.0221 
x.change 0.02597
```