removing example-models repository

#### Summary:

Removes `example-models` from Stan as a submodule.

#### Intended Effect:

- makes it easier to track Stan. no need to deal with the `example-models` repo from within Stan.
- `example-models` should be included as a submodule by each interface separately.
- doc updated to indicate models are in the example models repository.

#### How to Verify:

The `example-models` submodule should no longer be tracked.
Manual is has been changed.

#### Side Effects:

Each interface will need to include the example-models submodule.

#### Documentation:

`programming.tex` in the reference manual has been updated.

#### Reviewer Suggestions: 

Anyone.