Resource owner password credential grant optional refresh token

Currently, oauthlib does not allow `refresh_token` to be generated optionally for a Resource Owner Password Credential Grant.  

This pull request brings oauthlib up to date with its own docs for the [Resource Owner Password Credential Grant](https://oauthlib.readthedocs.org/en/latest/oauth2/grants/password.html) by allowing the `refresh_token` to be generated optionally.  Specifically:
  - I added the keyword argument`refresh_token=True` to the `ResourceOwnerPasswordCredentialsGrant` to govern whether or not a `refresh_token` is generated
  - Update the `ResourceOwnerPasswordCredentialsGrant` class's `__init__` docstring to reflect the change
  - Add a corresponding test in `test_resource_owner_password.py`