Moved example models to submodule

#### Summary:

Example models in `src/models` have already been moved into their own repository: stan-dev/example-models.
This pull request:
1. removes models from `src/models`
2. adds `example-models` as a submodule to this repository
3. removed `src/test/unit/gm/parser/models_test.cpp` since it isn't a unit test and it's only testing a few of those in the submodule
4. update docs that reference `src/models`

The links for the wiki pages that link to the Stan models have already been updated to point to stan-dev/example-models.


#### Intended Effect:

Allows the example models to be managed outside of stan-dev/stan.


#### How to Verify:

The models in src/models should have been removed and should all be in stan-dev/example-models.


#### Side Effects:

The example-models are now a submodule and needs to be updated as a submodule.


#### Documentation:

The references in the manual to `src/models/...` have been updated to `example-models/...`, which is where the same files will exist.


#### Reviewer Suggestions:

Bob. 

Questions: 
1. should we even include this as a submodule of Stan? Or should it be included as a submodule directly from all our interfaces?
2. what should we do with our out-of-date model tests in CmdStan? They only live in CmdStan and I think they should just be removed.