Feature/issue 491 add litters

#### Summary:
Fix issue #491  by adding litter BUGS example. Test is added in https://github.com/stan-dev/cmdstan/tree/feature/issue-491-added-litters.

#### Intended Effect:
Add litter BUGS example.

#### How to Verify:
Check model file and model test in cmdstan.

#### Side Effects:
None.

#### Documentation:
Added to wiki page (link leads to where the files will be once this is merged).